Myles Elton was 21 when his doctor told him he might never have children. He and Erin were teenage sweethearts and had married the year before. They’d planned to have children eventually, but not for a good few years yet. Then, after an offhand comment from his mother about a medical complication Myles suffered as a baby, which can often lead to infertility, he booked in for a series of tests. The results were life-changing. “They told me we had a 15% chance of having kids,” he says. “It was pretty heartbreaking, to be honest. In our minds, that was something that would just happen.”

The doctor encouraged Myles and Erin to start trying to get pregnant straight away; if they didn’t, there was a high chance they might never have children. “He said, ‘You’re young, that’s on your side. The most likely time you’re ever going to get pregnant is now – the longer you wait, the likelihood decreases.’” Erin and Myles discussed it for a while. “We really wanted to have kids and we thought, we could be trying for 10 years and still not have any. So Erin went off the pill.”

After five months, Erin was pregnant with Lucy, who will turn five in March. It took seven months to conceive Norah, who was born when they were both 24. Myles and Erin are now 27, and hoping for a third child.

The family live in Wollongong, an hour south of Sydney. When I speak to them, the girls have just had bathtime and are sitting, damp-haired and pyjama-clad, on the bed with their dad. Lucy is a bundle of giggly, nervous energy, and with good reason: it is the day before she is due to start school and she is excited. She doesn’t know her teacher’s name, but tells me her uniform is blue, her lunchbox is pink and she is most excited about seeing if her classroom has a particular toy that uses coloured magnets to make a picture.

“And learning how to read,” Myles says. “She can write her name, and she remembers the stories from the pictures, but she’s really excited to learn how to read.

“We’re not sure how we’ll go. Will we cry? Will she cry? It’s a big parenting moment,” he says.

One thing is sure: they will be among the youngest parents at the school gates. Myles says that when people found out he and Erin were expecting Lucy, they were shocked. But he doesn’t regret becoming a young father. “When it’s something you thought you might not ever have, it’s such a joy. I love being a young dad. I’ve never been an old dad, so I don’t know the difference, but I’ve got lots of energy and heaps of time.”

He and Erin were both students when Lucy was born: “We didn’t have full-time jobs, we didn’t have a house, it wasn’t like we were set.” But they have managed. “We’re not going on holidays to Europe, we don’t buy new cars, we won’t buy a house any time soon. We’ve had kids at the time when I might have been earning lots of money, and I’m not. But there’s plenty of time to earn money – there’s not plenty of time to have kids. It’s now or never, at least for us.”

In many ways, Myles has been lucky. He found out he might be infertile in plenty of time. Many men won’t realise they can’t have children until it’s too late.

***

Professor Allan Pacey has worked in the field of male infertility for two decades. What many people don’t realise, he says, is that a man’s fertility, like a woman’s, has a shelf life. “I don’t think men have a biological clock like women do, in the sense that there’s no male menopause, but fertility in men declines far more sharply than people think,” he says. While much of the public discussion around fertility relates to women, the reality is that male infertility is a factor in about 50% of cases when couples struggle to conceive, and is the sole factor in 20-30%. As with women, fertility issues increase as men age. This news is difficult to comprehend for a generation of men who have grown up seeing images of famous older men becoming fathers.

Charlie Chaplin’s 11th child, a son named Christopher, was born in July 1962 when the actor was 73. Rupert Murdoch’s sixth child, Chloe, was born when he was 72. Pablo Picasso had his last daughter at the age of 67, the same age Steve Martin had his first, and Rod Stewart, who had his first child at 18, welcomed his eighth at the age of 66.

But these are exceptions to the rule. While some men may be able to father children well past retirement, Dr Channa Jayasena, a reproductive endocrinologist working at Imperial College London and Hammersmith hospital, says these cases are rare and “give men an inflated sense of their own fertility”.

“I think it makes men feel that we’re slightly immortal when it comes to fathering children,” Jayasena says. “We have the idea that women have this body clock and they’re desperate to capture a man and settle down, but men can go forth and rise to the top of the career ladder, then settle down and have children, then maybe have a second relationship and a second family later on.”

In reality, Jayasena says, once men are over 35, their age and the lifestyle changes that come with it become major factors in whether their partner will be able to get pregnant. Research now suggests that men above the age of 35 are about half as fertile as men under the age of 25. After adjustment for other factors, including maternal age, the probability that an ultimately fertile couple will take more than 12 months to conceive almost doubles from 8% when the man is under 25, to 15% when he is older than 35.

The decline in fertility is not as clearcut among men as among women, with experts debating the relative influence of contributing factors, including reduced sperm count and motility, and lifestyle changes such as decreased sexual frequency and increased bodyweight. But a 2015 study from the University of Otago in New Zealand, which conducted a review of the data and findings of 90 studies of male fertility, found the evidence to be “robust” that age causes a decline in semen volume and motility, and an increase in the fragmentation of sperm cells. Whereas women’s fertility drops sharply around their mid-30s, for men it is a more gradual decline that begins earlier, with the odds of conceiving within six months decreasing by 2% a year from around the age of 25. A 2006 review of the literature for the American Society for Reproductive Medicine suggested that 40 could be considered the “amber light” in the reproductive life of men, as 35 is for women.

Advanced paternal age also leads to an increased risk of miscarriage. In 2008, researchers led by Dr Stephanie Belloc from the Eylau Centre for Assisted Reproduction in Paris studied 12,000 couples undergoing fertility treatment. They found that the chances of miscarriage almost doubled, from 16.7% for 30-35-year-old men to 32.5% for men over 40, irrespective of the age of the mother.

Yet unlike women, whose biological clocks are endlessly discussed and dissected, and who are now in some cases being offered egg-freezing as a corporate perk, young men tend not to think about their fertility unless something goes wrong.

***

Until five years ago, fertility had not even crossed Harry’s mind. The 27-year-old from Essex had suffered from pain in the scrotum for five years, and two years ago was diagnosed with varicocele, an abnormal enlargement of the veins which affects blood flow to the testicles.

“To be honest, I’d never really heard about men having problems with fertility, but I’d heard about women,” Harry says. Then he was faced with the prospect of surgery, which carried a slight risk of affecting his fertility. The decision was easy, he says: at the age of 26, he froze his sperm.

Harry clearly finds the subject uncomfortable. He struggles to say the words “sperm”, “testicles” or “scrotum”. But he is completely comfortable with the choice he made. “It’s very important for me to have children,” he says. “It’s a big thing. And I’ve got peace of mind, knowing the sperm is frozen. I can have more operations, or if I have injuries, it’s just there as a backup.”

Harry pays about £300 a year to have two samples sitting in a freezer at a clinic in London for up to 10 years. The process of having his sperm tested was revelatory: he has, according to the clinical scientist who froze his sample, “excellent sperm”.

“They gave it a score,” he says. “It makes you more aware of the quality of sperm. You don’t have a clue what it should or shouldn’t be, the size of them, shape of them, how many per millilitre.”

Harry discussed his decision with his family and co-workers, with whom he is very close and who kept asking where he was disappearing to when he went to appointments, but he has not told any friends outside of work. “It’s not spoken about,” he says. “Maybe it should be. You might have some blokes who get to 35 and realise they can’t have children, but if there was some testing going on and they knew there was a deterioration…”

Will we be seeing more young men freezing sperm for their own use later in life? Dr Sheryl Homa, scientific director at male fertility clinic Andrology Solutions in London, says that while many of the taboos about male fertility have disappeared, much of the discussion is still directed at women. She cites cases where male patients have told her they have gone to see an IVF doctor only to have the doctor speak exclusively to their female partner, or how at many fertility clinics they have personal stories of infertility struggles displayed on the walls or website. “And they’re always the women’s stories,” she says. “It’s a very lonely place for men.”

***

Nick Black, 28, a freelance animator from London, might fit many people’s notions of a millennial man. He is ambivalent about marriage, none of his relationships in the last five years has lasted longer than a month, and he has no intention of having a child any time soon. “Because, when you have a kid, your whole world has to change,” he says.

But, despite having set an age limit of 40 in his mind – “Beyond your 30s you’re too old to keep up with the kid. If you’re 50 and the kid’s 10… you’ve missed the boat” – he is unfazed by questions about when he might reproduce. “As a man you feel like you’ve got this reservoir,” he says.

In many ways, Black is typical of the men of his generation. In 2014, Generation Y women – aged 20-35 – produced 76% of the country’s babies, while Gen Y men were responsible for just 62%. The average age of fathers reached a record high of 33.1 in England and Wales in 2014, compared with 32.6 in 2011 and 30.8 in 1991. The men in the generation biologically best placed to procreate are putting off fatherhood.

According to Dr Anne Goriely, associate professor of human genetics at the University of Oxford, we need to be more aware of the limits of male fertility. “We have a pretty good idea of what the risks are for women,” she says, “so the recommendation is, don’t leave it too long. I think it would be wise to give the same advice to men.”

As well as increased difficulty with conception, and fewer options when it comes to fertility treatments, there is a clear link between genetic disease and advanced paternal age. “A number of studies suggest that the quality of sperm declines gradually from the age of 30,” says Jayasena. “The older you get, the poorer the quality of genetic material you are transferring. The risk of having a baby with a genetic abnormality is low, but it’s certainly higher the older you are.”

“With women, we understand quite well the type of problems that occur with advanced age,” Goriely says. “With men, it is more difficult and the effects are more subtle. Except for mutations causing a few disorders that we know are strongly associated with paternal age, we don’t have a good understanding of what is going on.”

Goriely likens the problems caused by a man’s age to “spelling mistakes” in a copying process, but these errors can accumulate in serious ways. “If we want to consider the father’s age specifically, we can estimate that a man in his 50s contributes an extra 0.5% of risk [of a serious condition],” Goriely says. “So the individual risk associated with paternal age is low, but for every man reproducing in their 50s, they will each contribute to 1 in 200 extra births with serious health problems.”

The amassing evidence that older fathers are more likely to pass on congenital abnormalities to their children was cited by the Human Fertilisation & Embryology Authority in 2011, when it announced the decision to reduce the upper age limit for sperm donors in the UK from 45 to 40, despite an under-supply of donors across the country. “The worry is that maybe over several generations the effect could be accumulative,” Goriely says. “If you systematically reproduce while progressively accumulating small genetic abnormalities, you’re loading the genome with a burden that can result in serious health problems.”

***

Steve first visited a fertility clinic in October 2014, when he was 42. He and his 41-year-old wife Ellie had been trying to conceive for a year. “They said there was nothing wrong with my wife – it was me,” Steve says. “A urologist said, ‘You’ve got no chance of having children,’ which is pretty devastating, to say the least.”

This appointment was the first in a year-long process of fertility treatments that cost the London couple between £25,000 and £30,000. At one point, Steve flew his brother over to make a sperm donation, in case all else failed. But before reaching that last resort, Steve flew out to see an expert in San Francisco, who recommended having his testicles “mapped”.

The mapping procedure involves a doctor inserting 18 needles into each testicle, to discover whether there are any reserves of sperm. It was far more accurate than the other procedures Steve had undergone, and the San Francisco doctor found some sperm. Steve returned to the UK where it was harvested and, after one round of IVF, his wife became pregnant. She gave birth to their baby boy early last month.

Steve is thrilled, but still angry at having been given such a definitive, and ultimately wrong, initial diagnosis. He puts it down to the more general lack of information surrounding male infertility. “In most cases you hear about, it’s the woman who has the issue. But if you look at the statistics, it’s men as well. They don’t tell you that.”

Steve can never know how much of an impact his age had on his sperm count and motility, but thinks the treatment would have been more straightforward and far less expensive if he and his partner had been younger. “If we were 10 years younger, we could have waited a year to give the medication time to work, and I wouldn’t have had to go through the whole operation.”

Just a week after Ellie and their son came home from hospital, and despite a fog of sleep deprivation, Steve says the expense and the invasive procedures were “absolutely worth it”.

“It’s hard to put into words when you walk out of the hospital,” he says, “but flying overseas, being poked and prodded and cut open – it means nothing now, compared with what we have.”

• Some names have been changed.