For as long as I can remember, I’ve wanted to be a grownup. As children, my friends and I would play at being shopkeepers and customers, thrilled to inhabit an adult role. As a teenager, I lived alone abroad. By my 30s I had all the things I thought signalled adulthood: a career (as a journalist), a home, a husband, a washing machine, a dishwasher and a fridge. All the paperwork and white goods to prove I was finally the competent, confident adult I had always hoped to be.

But at random moments my non-adultness would pop out, like when I opened my kitchen bin to find the lid thick and throbbing with squiggly maggots, and immediately called my mother for advice. Or when my bag was stolen and people suggested my contents insurance might cover it. What contents insurance? While I was training to be a psychodynamic psychotherapist, and as a patient in therapy myself, I had more and more of these moments where I felt so unknowing and lost. Officially, I was a grown-up thirtysomething, but at times like this my adult skin felt paper-thin. In these moments, not only did I not feel like an adult; I realised I didn’t even know what one is.

To find out, first I interviewed older adolescents, and world experts in the neuroscience, evolutionary biology and psychoanalysis of this life stage. Then I investigated young adulthood, parenthood (or not) and middle age. At every step, most people told me they felt they had a lot of growing up still to do.

Finally, I came to old age. I confess, I thought all older people must be fully cooked adults by default. I fell into the trap of assuming that, if you’ve grown old, you’ve grown up. It’s comforting to believe questions of being an adult, of what even is an adult, have been answered by someone who draws a pension. That, says Pat Thane, author of The Long History of Old Age, is “the stupidity of assuming that, past a certain age, everyone is much the same and has the same sort of experiences. But they don’t.”

People are much more interesting than that.

***

Whether we’re talking sociology, medicine or neuroscience, old age is said to begin around 60. I’m surprised to learn from Thane that our idea of “old” hasn’t changed much in hundreds of years: in medieval England, people were liable for compulsory work until 60; in the crusader kingdom of 13th-century Jerusalem, knights over 60 were exempt from military service; today, people get their state pension between 60 and 70.

What has changed is that last century, it became normal to grow old. This was thanks in part to a new specialisation, geriatrics, a term coined by Austrian-American doctor Ignatz Leo Nascher in 1909. Nascher, Thane says, “believed doctors paid insufficient attention to the ill-health of older people: as they had not long to live, it was not thought worthwhile trying to cure them”. In 1948 came the NHS, better living standards and food; now more of us grow older, spending more of our old age in good health, though health inequalities remain stark.

It became normal to grow old, but not normal to like it. Why else do we have anti-wrinkle creams; “60 is the new 40”; “old” as an insult and a youth-supremacist society? Sarah Lamb is professor of anthropology at Brandeis University in Waltham, Massachusetts, and author of Successful Ageing as a Contemporary Obsession. “When people say they’re old, the connotations in our public culture are that you’re less vital and open to growing, and have stopped learning,” she says. We’re so steeped in this attitude, we might assume it reflects an objective reality rather than revealing something about our psychology. As Lamb notes, this paradigm “is not the best, most humane or inspirational way to imagine ageing and what it means to be human”. What if older people are just as clueless about growing up as the rest of us?

That seems to be the case, at first, for Graham. He recently retired from academia where, he says, “it was very important to put on a front”. He had to appear authoritative and resilient to survive, overcoming the part of him “who is quite vulnerable, shy and reticent”. For four decades, he says, “I was constantly overriding the person I really was.” And now – are you grown up? “Not really. I’m a month short of 66, which is grown up in numerical terms, but I don’t feel grown up at all.”

Graham spent his childhood in Bootle, a working-class area of Liverpool, Merseyside, surrounded by a large extended family. Asked to share any early memory, he describes a present received age three, when his sister was born: “It was a plastic guitar. I found that inspiring – music was all around us, when the Beatles were taking off.”

In his mid-30s, Graham took all the adult steps – buying a house, marriage, fatherhood – but the marriage broke down. He devoted himself to his daughter and work but says: “I wasn’t in the land of the living.” Hope came in his 50s, when a local Irish folk centre advertised for violin players. He’d not played since school, but picked his up again and found “it was remarkably evocative, the feeling, that texture. There’s something very tactile about musical instruments, and there’s an excitement. What sound can you get out of this?”

As an adult, Graham had forgotten how to play and didn’t think he would persevere, but he has – for years. After moving back to the north-west, he plays with Yorkshire Late Starters Strings, a charity offering adult beginners the chance to learn and join a community. This has opened up new possibilities for Graham to enjoy a different relationship with others, and with himself – a later-life growth spurt he desperately needed. He made new friends, grew in confidence. “I felt a different person, more comfortable with myself. I got to the point where I thought: I’m just going to be who I am.” At 63, on holiday, he met a woman who is now his partner. “There’s a kind of Indian summer effect, where there’s not so much pressure to perform or be anyone,” he says.

After being diagnosed with type 2 diabetes, he began living more healthily, and went into remission. “That’s a very important thing about growing up – realising your mortality. You can’t just override physical problems, you’ve got to take care of your body.” Deterioration is inevitable, he says. “A lot of people don’t want to accept that; they want to think they can just keep going. But I’m not the same person I was at 22.” It’s not easy to accept these changes, partly because of how society treats older people. “The negative element to being this age is you feel marginalised and quite redun –” he coughs, as if the word sticks in his throat, “redundant.”

Music helps him feel he’s making the most of his time: “It’s absolutely in the moment. There are very few things like that.” His 30s, 40s and 50s were always about deferred gratification – something was going to happen, some article was going to be published, or a grant won. “That was endless. You have to arrive at a point where you enjoy what is happening right now.”

Everything he says sounds very grown up to me; I wonder if it does to Graham. He says, “It’s hard, because I still don’t really know what growing up means.” He’s not the only one. He reflects and adds, “If growing up is getting to be at peace with yourself, that has happened.” As definitions go, it’s the best I’ve come across.

***

Research by Prof Klaus Rothermund, chair of general psychology at Friedrich Schiller University in Jena, Germany, shows older people are more interested in supporting others, in using their energy for the benefit of younger generations, and in moral standards. While young people tend to choose long-term aspirations, acting now to reap future benefits, these strategies lose meaning in old age, so people choose goals “where at the moment you act, you already see the meaning of your action”, Rothermund says. His experiments show people of all ages change their goals when they imagine their lives might end imminently. “The crucial thing is, if time is limited, that immediately makes you aware that you cannot do everything. You focus on what is most important.” If we’re lucky, when we grow old we experience this “developmental achievement … living in a way that focuses on what’s truly, lastingly important.”

When I saw a viral video of an elderly rabbi carried by police at an Extinction Rebellion protest in 2019, I wondered what he might have to say about growing up in this final phase of life. What does it feel like for Jeffrey Newman, 78 when we speak? He had been scared of his 70th birthday, he says. From 70 on, “you can’t escape the fact you’re elderly.” I ask what difference it made and he says drily: “Nothing. The good thing about going through these barriers sometimes is to come out the other side and realise, actually, it’s just another day.”

For Newman the change started at 59, when he stopped working at a synagogue, giving him freedom and time “to work out what I really care about”. Since then, “It’s been a very exciting period of my life, learning even more about living in the moment and discovering what it means to be of service … Eldership has been lost in the west,” he says, “and it’s essential that we rediscover it.” He doesn’t have a clear definition – he’s still working that out – but after helping to form an eldership group in Extinction Rebellion and meeting with elders from across the world, he mentions curiosity, kindness, awareness, sensitivity and courage. “It’s got something to do with wisdom. That’s why you wouldn’t ever say or think you’re an elder, because it’s also got something to do with humility. All you can know is that you’re working towards it, not that you’ve ever got there.” With a sigh, I understand the same is true of growing up.

Newman speaks of his arrest at that protest as “an initiation ceremony” into elderhood. It was an act of service. He was afraid, he says, but “it was about being prepared to step out”. It’s a curveball to learn that, for a 78-year-old rabbi, a key milestone of growing up was getting arrested. But as I let go of the idea that I’ll ever arrive at a clear definition and state of adulthood, I seem better able to embrace its contradictions and paradoxes.

***

If I need any further proof that old age does not necessarily correlate with adulthood, I find it in a woman called Pog. There is a bounce and rhythm to her speech that feels determined, stimulating, playful; it’s fitting that her nickname is a shortening of pogo stick. “Now that I’ve started thinking about it, I’m quite shocked really,” she says when we speak. “I truly do not consider that I have grown up. And I’m 90.”

Pog was born in Malta, where her father served in the navy. When she was nine, he was killed when the submarine HMS Thetis sank: “It affected my mother’s mind, waiting three days to see if they would get them out.” When she was 13 her mother was sectioned and Pog was adopted by a cousin. These losses, she says, “made me extremely grateful for the stability I subsequently found in marriage and motherhood.” She met her husband, Brian, at church and they had three children. “The kids as teens, oh God, they were menaces, rebels. But they were such fun, and they still are.”

Pog feels she grew up most through coping with her late husband’s Alzheimer’s. He was diagnosed when she was 78. “That’s when I learned some very unpleasant things about myself. I lack patience, I lack empathy. Everybody says, ‘Oh, you were marvellous with Brian.’ But I know the truth. And that was serious growing up.”

They lived with Brian’s diagnosis for eight years, and Pog became his carer. She didn’t want to move him to a home, but their children were worried about the strain on her. “Towards the end, I wasn’t able to get out at all,” she says, except for a weekly hairdo – her therapy. “I’d only be out for an hour, then I’d dash back.”

Brian lived at home until he died, and Pog speaks honestly about the confusing mess of feelings she was left with: “The awful thing is I was so grateful for his death.” Grateful not to have to move him, and not to have to cope any longer: “It was a ghastly state to be in. Anybody looking after somebody with Alzheimer’s is going to have some really bad moments.”

Eighteen months later, the grief hit. “Suddenly everything changed,” Pog says. “I was crying at sloppy feelings and tunes and stuff like that, having memories. I truly mourned.” What followed was “a complete softening” of her personality. “Until then, there’d probably been a shell that was put on for outside viewing.” Pog had always been the person keeping it all together, not realising that she was “putting on a show” – until she wasn’t any more. When a friend recently signed his book “to Pog and Brian”, because that’s how he still thinks of them, she had a “total meltdown of grief”, six years after his death. “It makes me happy there are still chinks in the armour,” she says. Listening to her speak, I understand how mourning can lead to a renewal of life, to a deepening of our understanding of ourselves and of others, and, at every life stage, to the most meaningful kind of internal growth.

“I do think I have grown up as far as that’s concerned,” Pog says. “But for God’s sake, how old was I? 85!” And she still doesn’t consider herself a real adult. “There’s still a long way to go. The real sort of inside growing up, the mental attitude, I don’t feel I’m there yet.”

Rather than telling herself off about this, she sees its value. “One thing that really pleases me is the childish pleasures. Where you clap your hands and say, ‘Oh! Look at that!’ Somehow, I’ve still got that, and I love it.” This brings to mind Graham playing his violin, Rabbi Jeffrey discovering how to be of service – it is a choice to come alive and to continue coming alive, to grow throughout life. I ask if she thinks she will finish growing up in her lifetime. She says she hopes not, “because in a way, that’s a sort of immortality, isn’t it? While I may accept the fact I’m going to die and all that stuff, if there’s still a tiny spark that says there might be something more to learn, isn’t that something to look forward to?”

• When I Grow Up: Conversations With Adults in Search of Adulthood by Moya Sarner is published by Scribe at £16.99. To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.