Adam Harris is 16. He has wanted to use a gym for years but his mother, Diana, stopped him. 'She was frightened it might stunt my growth,' he says now. 'So I did every other sport I could: football, rugby, kickboxing. I jogged all the time and had a set of weights in my room. I wanted to look like Justin Timberlake and all those other men the girls like.'

This summer Adam finally found a gym that would let him exercise, although it won't let him join as a full-time member until he is 18. His body shape is testimony to the fact that over the summer, Adam, along with three friends, spent three-and-a-half hours every day in intensive workouts.

'I originally went because I wanted to impress girls, but after exercising I've got no energy to do anything except come home and lie down,' he admits. 'It's not about girls any more; it's about the exercise itself. The gym encourages you to be self-obsessed and critical. I could easily have become really obsessed with exercising; it was a close thing I didn't.

'Gyms think out every detail to keep you going: they have all these magazines lying around with impossible images of men on them and even arrange their lighting so it's more flattering,' he says. 'When you get home and look at yourself in the mirror, you get all downhearted and want to rush straight back.'

For generations it has been young women who have been made to feel insecure about their body image. Increasingly, however, men - and boys - are being bombarded with images of perfection. Half-naked men with bulging biceps, glistening pecs and rippling six-packs grace magazine covers, cinema screens, album covers and the catwalks. Men are being left in no doubt about the shape they have to be in to be beautiful, and the quest is starting ever younger.

Amir decided he was fat at the age of 11. He'd never been particularly aware of his body before but suddenly he began noticing the men on the front of the magazines his older sister read. 'I remember looking at their muscles and their stomachs, and they just looked so different to me,' he says now, four years later. 'The more I started looking around me, the more images I saw of men with muscles and six-packs. I remember comparing my soft and chubby one to theirs and feeling terrible. I knew they were older than me but I couldn't see how I could ever look like them if I didn't do something right away.'

So Amir started smoking. For a year he had a secret cigarette before every meal to curb his appetite and then another afterwards to blunt any hunger pangs he might still be feeling. 'I hated it,' he says. 'I could feel it doing me harm and I didn't like smelling of smoke the whole time. All my friends thought I was stupid, and I didn't like lying to my mum.'

Despite his unhappiness, Amir continued smoking for a year until he had shed the stone in weight he had decided to lose. Then he started exercising. 'I was too young to go to the gym so I persuaded my mum to buy me some weights and I used them every day.' He did 80 sit-ups every morning and the same number of press-ups. He lifted his weights for an hour a day and became obsessed with watching his body change. 'I looked like a real man when the rest of my friends still looked like little boys,' he says, still proud.

His pride subsides, however, when he holds out his arm. 'I can't straighten it any more,' he says. 'It's because I lifted too many weights when I was young. I tore my ligaments and they fused back together wrongly.'

There is another problem too: Amir is stocky and less than five foot tall. Both his older brothers are over six foot tall and slender. 'I think I stunted my growth because I worked out so much,' says Amir. 'All I wanted was to be like those men in the magazines the girls want to be with.'

Boys' insecurities about their appearance hit a peak during adolescent years when the growth spurt makes them look tall and skinny. In girls, puberty is often seen as a negative event that brings weight gain and curves. In boys, however, it is a different story: it's an outward sign that they're becoming bigger and stronger. 'Boys have always had role models in celebrity and sporting heroes, but those idols have never been so uniform in the way they look and so happy to strip off and reveal all,' says Lee Miller, an expert on child and adolescent mental health with the Young Minds charity. 'There is now just one image of strength and success for young boys and it focuses entirely around that ridiculously muscly body and washboard stomach. It's no wonder that boys are becoming painfully aware increasingly early if they don't measure up.'

While girls diet, starve and purge to rid themselves of their curves, boys go in the other direction, indoctrinated, Miller believes, with the idea that fitness, strength and health go together. That process is starting ever younger in the shape of gyms such as Nova at the Next Generation Club, the first health and fitness club in the country to offer children's gym facilities. Aimed at children between two and 14, the centre in Dartford has developed a range of gym equipment for children, including cardiovascular and weight machines. 'I'm trying to build up my arms because I'm a bit skinny,' says Curtis Horne, aged 12, who has just arrived in the Dartford gym straight from school. 'I usually go on the treadmill to warm up then work my way around until I've done everything.'

'I come after school and at the weekends,' says Luke Perry, eight, shy and blond. 'My favourite part is the weights, but you have to do your warm-up before you go on any of the machines.' He exercises for 40-50 minutes and says: 'I think I'm getting fitter.'

Malcolm McPhail, who helped develop the machines, says: 'Research shows that from the age of seven children can benefit from using scaled-down training equipment. It has muscular-skeletal benefits and a positive psychological effect.'

Gym manager Julie Bartlett adds: 'How many kids are at home in front of the telly at 5.30 on a Friday night when they could be exercising?'

The problem comes when boys not only get extreme ideas of what they should look like but of how they can achieve that look. UK Sport, a government-funded agency that represents sports governing bodies, has been contacted by parents of children as young as 11 who have found muscle-boosting products, including steroids, in their children's possession. Steroids increase muscle mass, reduce body fat, produce quicker results from gym workouts and also give users a mental boost, making them feel more popular and sexually attractive. They also stunt growth and the overdeveloped muscles they promote damage ligaments and tendons. Even slight overuse can result in excessive acne, immune disorders and mental problems such as depression and aggression. Long-term use has been linked to heart and liver damage, infertility and prostate cancer.

Although they are illegal, steroids are none the less easily obtained. A survey of 1,000 teenagers conducted recently by Dr Robert Dawson, a GP in Chester-le-Street and director of the NHS-founded Drugs in Sport Clinic, found steroids were the third most common drug offered to 14- and 15-year-olds after cannabis and amphetamines. Another survey found that around half of GPs in some areas of the UK have seen steroid use in their surgeries in men aged from 14 to 60.

Boys believe taking steroids is no more harmless than smoking a joint, Dawson says. 'Part of the problem is that you hear so much about steroids these days that young people assume they are OK. If a top sports person is using drugs then how can they be unhealthy? It all reinforces the reasons they need to take them.'

But is it such a bad thing that boys are becoming increasingly aware of their bodies and their health?

Since the mid-1980s the number of children and young people classified as being either overweight or obese has leapt enormously. Between 1993 and 2000 the percentage of men aged 16 to 24 classified as obese grew from 4.9 per cent to 9.3 per cent. Even worse is the recent finding that one in five four-year-olds is overweight. The effect of this change in the national body shape is deeply worrying: obese or overweight children may not outlive their parents because of the greater risk of developing premature heart disease and certain cancers. Young overweight people have begun suffering a form of diabetes that has, until now, only been diagnosed in older people. The problem continues into adulthood, with one fifth of British men now classified as obese and half overweight, a statistic that has increased by over 60 per cent in the last decade, according to Cancer Research UK.

Experts fear we are slipping into a society of extremes. While on the one hand, some boys are endangering their health through a complete disregard for their bodies, others are going to great lengths to achieve their toned muscles. 'I'm really aware of how I look,' says 13-year-old Patrick Healy, from north London. 'My friends eat all sorts of junk food but I only drink one can of fizzy drink every couple of days, and the rest of the time I try to stay really healthy and eat things to bulk me up.'

US trends might offer some clues. A survey of 1,000 teenagers questioned by Dr Katharine Phillips, a psychologist at Brown University, Rhode Island, found that when presented with a range of images of male physiques, more than half of those questioned wanted a body shape that could only be achieved by using steroids. 'I don't think I had body dysmorphia or anything, but every time I looked in the mirror from the age of 11 to 14 I saw a puny little boy,' says Amir.

Six months ago Amir got a girlfriend and has now reduced the intensity of his exercising. 'I regret the amount of time I used to spend working out, and I try not to think about what I might have done to myself,' he says. 'But at least I didn't do any of the strange diets I could have done or take drugs; I could have been really excessive.'