My personal trainer and I are always getting into arguments about what part of my body needs the most work. I'm not happy with my abs – I have the remains of a small spare tyre – but she says my bottom is a catastrophe because it's so flat. What we both agree on is that bodies can be remodelled, no matter how old you are.

I was a very sickly child. From the age of six I had constant headaches and chronic tonsilitis. I became pale, sluggish and my growth was slow. I remember noticing one day that my best friend, who was a year younger than me, was slightly taller and that I was very upset about it.

At 13, I had my tonsils removed and as my health improved, everything changed. I shot up and suddenly I was full of energy. I thought back to myself as a frail, sickly boy, and vowed never to be like that again. I took up boxing, rowing and rugby. Staying fit and strong became my priority.

After school I trained to be a dentist, but sport remained an important hobby. I only once let myself go. As I crept into my 40s, I adopted my wife's sedentary lifestyle. We spent a lot of time doing nothing. Inevitably, my blood pressure plummeted and one day I felt a sharp pain in my legs – only to discover the dark, earthworm-like patterns of varicose veins across my calves. It was my first brush with old age, and I didn't like it. Immediately I resumed rowing to stay fit.

Life went on. My wife and I divorced. At 60, I discovered veteran's rowing and started competing internationally, eventually winning 36 gold medals. I'm not a particularly talented sportsman, but I've always been a great trier. At 75, many of my friends began to pass away. People were getting older around me, but I was only just ready to retire. I carried on rowing and publishing a dentistry newsletter until I was 82.

Then at 85 I had a crisis. I looked at myself in the mirror one day, and saw an old man. I was overweight, my posture was terrible and there was skin hanging off me where muscle used to be. I looked like a wreck. I started to consider the fact that I was probably going to die soon. I knew I was supposed to slow down, but I'm vain. I missed my old body and wanted to be able to strut across the beach, turning heads.

I was already rowing six times a week, and there didn't seem any harm in pushing myself a bit harder to rebuild my muscles. So in my late-80s I joined a bodybuilding club.

There's no research into bodybuilding for the over-80s, so it's been an experiment. With weight-lifting and protein shakes, my body began to change. It became broader, more v-shaped, and my shoulders and biceps became more defined. People began to comment on how much younger I looked, and my new muscular frame drew a lot of admiring glances from women.

Everything I learned was tailored to help my body cope with old age. I took up judo to teach me how to fall properly. My circulation and posture improved, and I was told that there was a chance more muscle mass could protect my brain from Alzheimer's. I stopped thinking about dying. As I approached 90, my focus was on getting my body back.

In 2008, I signed up for my first championship. I was nervous, but although I was the oldest contestant by around 20 years, everyone was very welcoming. I got higher scores than all the women taking part, and a lot of the men. Then, at last year's event in Germany, I triumphed, scoring higher than any contestant in any age category for my 57 dips, 61 chin-ups, 50 push-ups and 48 abdominal crunches, each in 45 seconds. As I'm over 70, they did make allowances – I could do the push-ups on my knees, for example – but I proved I wasn't past it.

I'm not chasing youthfulness. I'm chasing health. People have been brainwashed to think that after you're 65, you're finished. We're told that old age is a continuous state of decline, and that we should stop working, slow down and prepare to die. I disagree. To me, a 65-year-old is young. I turn 92 this year. It is a frightening prospect – the law of averages is against me, and, yes, one day something will happen and that will be it. But until that day comes, I'm going to carry on working on my abs.

• As told to Rhianon Howells

Do you have an experience to share? Email experience@theguardian.com