'The first 40 years of life is text, the rest is commentary," wrote Schopenhauer. Setting the watershed as low as 40 is arguable, but Schopenhauer surely had a point, and it may help to explain the results of a new survey that puts our most depressed age at 44. This vast study, carried out jointly by researchers at Warwick University and Dartmouth College in the US, has concluded that happiness is U-shaped: it peaks when we are 20 and 70, but slumps in the middle. "You would expect people to get unhappier as they get closer to death," says Professor Andrew Oswald of Warwick University, "but the opposite appears to be the case. It is a mystery why this happens."

If we trust Schopenhauer, it is no mystery at all. Your 40s are the point at which the act of composition - climbing the career ladder, having affairs, believing you are the next Montaigne - is replaced by the art of reflection and perhaps regret. How did I fail to become prime minister? Why did I have those affairs? Where is my oeuvre? At 44, those thoughts - as I painfully recall - are uppermost in your mind, and sometimes you will blame anyone but yourself for your failures. But trust me, you will come through it. I reached 50 last year, and far from being distressed by that supposedly defining moment, I've never felt better. I now accept that I am deep into my commentary period, and am enjoying it hugely.

In your 20s and 30s, you think there is some big secret that is being withheld from you. But there is no secret. No one has a clue what they're doing or why. By 44 you are distressed to discover there is no secret and that life's glittering prizes are made of tin. But then comes the getting of wisdom. As Oswald observes, "When you get older, you've learned to accept yourself." You aren't Montaigne, you aren't going to be PM, you are just you. In Schopenhauerian terms, will is replaced by art, acceptance and a sense of the universal. You learn to enjoy the comedy of life's struggle, and happily take your place in this huge and leaking lifeboat.