The rock star Bryan Adams sums it up with the title of his new album, 18 Till I Die. Adams is fretful about passing life's halfway point. "I'm still sort of moving around all the time, interested in seeing as much as I can before I'm 40. I have this curiosity, this adventurous spirit, and I just want to do everything I can." He is certainly in an adolescent mood with such tracks as I Want to Be Your Underwear. But is it better to pursue eternal youthfulness, or to grow up a year at a time, thus avoiding having a crisis every time a major milestone is reached?
Adolescence was never so prolonged. In her best-seller, New Passages (HarperCollins), the American writer Gail Sheehy argues that "people today are leaving childhood sooner, but are taking longer to grow up, and much longer to grow old. Like so many of my generation, I used to find it impossible to envisage myself at 50." Sheehy believes that we do not become wholly adult until we reach our "middlescence" at 45. She paints a complex picture of a range of internal ages and different psychological tasks to deal with at different stages.
While most of us feel internally younger than our years, some people feel older, or merely out of kilter at different times of their lives.
The actress and singer Petula Clark, who, at 62, is currently setting the stage alight as Norma Desmond in Sunset Boulevard, appears tireless and projects a youthful image. By contrast, the historian Dr David Starkey, a regular panelist on Radio 4's Moral Maze, 11 years Ms Clark's junior, has an aura of gravitas and has often been accused of being old before his time. Both, however, were serious and precocious children.
Petula Clark was an established star by the time she was 10, and her work brought with it adult responsibilities. "I never had a life like anybody else's. I was always monitored and never allowed to go out with boys." Meanwhile, she was earning a lot of money for the Rank Organisation, which wanted to perpetuate her little-girl image. "At 17 I was still in ankle socks but yearning with all my heart to become beautiful and mysterious like Ingrid Bergman," she says.
Dr Starkey grew up with an adult single-mindedness. "My upbringing made me middle-aged," he says. "I spent all my time being scholarly and reading books. I remember a friend of my tutor's took one look at me and described me as 'an old young man'."
In their twenties, these two personalities grew in opposite directions, one gaining genuine maturity, the other making up for lost youth.
Petula Clark moved to France, and there began "a period of grounding, discovering who I am. Having children changed me: I felt more in tune, doing something I was biologically supposed to do."
Dr Starkey's favourite character in history is Sir Francis Bryan, an adviser to Henry VIII, of whom it was reported: "No matter how old he was, he was young of condition." In that period it was more criticism than praise: although youth was envied, it was seriousness that really counted. Dr Starkey decided to take Sir Francis's advice. "I moved to London and left the Cambridge sherry-drinking set behind. I had been very tubby but lost three-and-a-half stones and emerged sylph-like, a butterfly out of a chrysalis. I became hedonistic, changing the way I dressed and behaved; coming to terms with being gay turned me into a prolonged teenager."
But how old do they feel inside today? "I don't know how a person of 60 should behave," says Petula Clark. "A lot of people I meet at my age don't look like me. I feel out of step, my clock is different. Many people are amazed by Sunset Boulevard and say: 'How can you do it? Climb all those stairs? And all those quick changes?' "
Dr Starkey, however, feels more or less his 51 years. "I've settled down into a serious relationship and feel that I have arrived. Therefore, in the past year or two, I've become nearer to my real age. I like to think of myself as young middle-aged, rather than older middle-aged. I still like to believe that I have some of the attractive qualities of youth, such as an openness to new ideas."
In our youth-obsessed society, you might want to believe that inside you can be 18 until you die. The problem is, how to pull it off without looking ridiculous? There is a line in Sunset Boulevard that destroys Norma Desmond: "There's nothing wrong with being 50 unless you're acting 20." Fifty does seem to have replaced 40 as the watershed age.
"There's nothing sillier than remaining young past 50," says Dr Starkey. "It's sensible to decide that 501s and tank tops are best left to others. I went to a party recently, and several people had not observed this rule and looked extraordinarily old. Mutton dressed as lamb is very ageing."
Petula Clark dresses for herself, and doesn't care what others think of her clothes. "I don't wear jeans, but I see no reason why I shouldn't. When I shop, I don't think: 'Am I the right age for this?' However, I do avoid bright colours such as red and yellow."
"Act your age," people say. But what does this mean? When we think of 40, 50, 60 we may visualise our parents or our grandparents. But how to be 40, 50, 60 ourselves? Dr Sidney Jones is a chartered psychologist and lectures at the University of London, on the second half of our lifespan. He feels that we are confused because of conflicting societal and historical influences.
"Our knowledge of what to expect of ourselves comes from watching others, and we're living in a marvellous time when everything is changing. Yet we have fixed pictures of ageing from our childhood images of our parents and grandparents. These expectations are so deeply rooted that we are constantly surprised. I once had a secretary who was a 39-year-old grandmother. She was a vibrant, modern woman, nothing like the picture we still hug of a little old lady by the fireside with a high-necked dress. The myths are always being exploded, but history takes a long time to shift."
Is it possible to grow old gracefully while holding on to the best qualities of being 18? I believe the secret is in looking forward rather than back. Norma Desmond thinks her audience is still out there, and that her past is in the future. She does not like what's going on in the world, so she locks herself away from reality. Growing old literally sends her mad. By contrast, although Petula Clark has sold 30 million records and has been a star for more than 50 years, she does not rest on her past success, but always looks for new challenges.
"Once something is done, it's done. I've been criticised for not being nostalgic about Downtown and all those Tony Hatch songs. I'm always thinking about the next thing, and that makes you eternally youthful."
You also need incredible self-confidence, a disregard for what others think, and perhaps the ability to lie about your age without blushing.