It's my birthday this week and any minute a card from London will drop on to the doormat. It's always the first to arrive; it will be tasteful and carry a greeting from the medical researchers who have tracked me since birth. It will also remind me that I'm turning 62.

Age is a terrible thing. Eight years ago I could easily balance on one leg with my eyes closed. OK, so I wobbled and eventually groped for a chair, but the nurse said I had stayed upright longer than anyone else she had tested. But when challenged again late last year, I staggered uncontrollably round the room.

"I don't think we can allow hopping," said the nurse, noting my abject failure on my file.

This information will be relayed to records held on me by the National Survey of Health and Development, the world's longest-running survey of its kind. It began with 13,687 babies born in 1946 and has been tailing me ever since.

It wins a mention in David Kynaston's book Austerity Britain, and was intended by its founder Dr James Douglas (one of only three directors of the study) to investigate the declining national birthrate. But once the war was over and the nation began breeding again, Douglas decided to follow 5,362 of us to study differences in health and survival among different social groups; more than 3,000 of us are still on the books.

The study's findings are now a unique record of the impact of childhood on later life: in the latest round of tests, they measured my grip, the strength of which has been clearly shown to be directly related to birth weight.

Similarly, the timing of the menopause is directly affected by childhood factors. "What we have shown in our study," says Professor Diana Kuh, current director of the ongoing survey, "is that various developmental and early life markers are associated with age at menopause. For example, we found that women in the study who were the lightest at two years old or who experienced parental divorce by age 15 had an earlier age at menopause than women who were heavier at two, did not experience parental divorce, and who had mothers with a later menopause.

"In contrast, we found that being breast-fed, higher childhood cognitive ability and increasing parity [the number of times a woman gives birth] were associated with a delayed onset of menopause. These results did not appear to be explained by later risk factors such as smoking, which we know brings forward the age of menopause. We are currentlyinvestigating whether early environmental or genetic programming may explain these associations."

Using your brain has, once again, been shown to be good for you. Survey members who went to adult education classes (or received job training) achieved higher scores in memory and word recognition tests than those whose education stopped in their 20s.

Other findings suggest that a woman's relationship with her mother may affect her mental health after the birth of her own children: those separated from their mothers for at least a few days in early childhood and who felt they received "a low level of maternal care" were at a higher risk of postnatal depression than other women. It also helps to be tall: loftier two-year-olds went on to have lower levels of cholesterol than their shorter peers.

However, the overarching finding of the work seems to be that if you want a healthy life, get a decent start. Mine seems to have been pretty good. In the early days, district nurses came to call frequently at our east London prefab and one recorded in 1950 that I was wearing a blouse and had had porridge for breakfast. I've now given up on the blouses but still have the porridge. The survey followed me through school - testing my brain, health and ambitions - and through university, and wouldn't let go when I was married and mortgaged. (One finding is that those who had a mortgage by 26 had a lower risk of early death. I borrowed my first £3,000 at 23, so I'm optimistic.)

Researchers visited my daughter, giving her at eight the verbal reasoning test they had given me at the same age. She did better.

As a child, membership of the survey made me feel a bit special and at times helped me get out of lessons I didn't much like. At university, I quite enjoyed the questionnaires, and now, as we subjects trudge into our early 60s, the survey is particularly interested in ageing. "The research already shows that what happens in early life affects what happens to you in later life," says Kuh. "The effect on the mortality rate is significantly different between those who have always had the best chances all along the way and those who have had the worst. There can be almost a five-fold difference."

"There are two groups among those who are doing well," she adds. "Those who have always had the good things in life, and those who are resilient: despite not having had the best things in life, they still seem to come out on top, and we are interested in what makes them resilient. But the number of people who went right from the bottom to the top is few."

The researchers recently put me through a series of thorough tests at the Wellcome Trust's clinical research centre in Manchester. They took phials of blood, made me blow down a cardboard tube, swabbed my saliva, weighed me, measured me, gave me an ECG and stuck a five-day heart monitor on my chest. They also gave my heart an ultrasound scan and laid me flat out to examine my bone and muscle quality and body lean and fat mass.

Then there were the questionnaires: had I had chest pains, leg pains, angina, speech problems? A stroke, diabetes, thyroid disorders? Did I cough in the morning? (No to all of those.) I walk a bit but had to ring a string of Nos to the questions about sport: I don't do any.

Now the results have come back. Blood pressure, cholesterol, heart are all pretty good. But it was a surprise to find that the whole body scan (if you ask nicely they will show you your own skeleton on the screen) showed that I have osteoporosis in the spine. I don't feel any aches and pains, but I'll be munching calcium tablets for the rest of my life.

They also tested my memory. They showed me 15 words and told me to write down as many as I could recall. I didn't do very well. They gave me the same test eight years ago and it would be interesting to compare results. But apart from remembering I was pretty good at naming birds and animals in 30 seconds, I can't remember much of how I fared in 1999. In fact, I'm not sure I can remember 1999.