My wife has a significant birthday today, one that ends in a zero. I won’t tell you how old she is, but if I did you could repeat the words she has been hearing in the last few days, leading up to the big day: “Really? You don’t look it. How do you do it?”

Instead I’ll reflect on the last big birthday that I had, a while back, when I turned 50. Things happened in the years between 40 and 50 that changed the way I look at the future. At 40 I had never thought, consciously, “How long have I got?” If pushed I’d probably have said that I might make it to 80, all being well, in which case my 40th birthday represented the halfway point of my life. In my 40s I started to wonder how long I might have left, prompted by things that I read and training courses that I attended.

I remember a piece in the Guardian looking at life expectancy. (I have been unable to find this piece online.) A journalist my age interviewed actuaries about their work and the calculations they make. Eventually he persuaded an actuary to commit to a figure, the life expectancy for anyone born in the same year as me who is still alive. Life expectancy for all men born in the 1960s is somewhere in the low 80s. Ten years ago life expectancy for all men born in the 1960s who were still alive was around 95. That’s the thing about life expectancy: the figure goes up the longer you live.

Around the same time some of the training courses I attended posed questions based on health and fitness. How long can a human body be expected to last? Is there a built-in timescale, similar to that of a budget car, which can only be expected to last 9 or 10 years before it becomes too difficult to maintain? If so, is it 90, 100 or 120 years? Or is technology going to change everything for us? If we are healthy enough (and wealthy enough) in 10 or 20 years’ time will we be able to take advantage of nanotechnology and gene research to maintain a healthy body for several decades, or even (theoretically) centuries? In this Guardian interview from 2009 Ray Kurzweil suggests that we will:

“People ask me whether I think taking all these supplements will allow me to live hundreds of years. No. The point is only to stay in good shape another 15 years or so before we have developed the ability to reprogram our biology through nanotechnology using nanobots – blood-cell sized devices in our bloodstream that will keep us healthy.”

 Even without this kind of intervention more people are living past 100 than ever before. How do you increase your chances of joining them? This can be a rather fraught subject. Many people, understandably, don’t want to think about it too hard. My mother died at 60, which seems young these days. Her mother died at 38 (or maybe it was 42 – there is some doubt about her exact date of birth), which has always seemed young. But even now, in some parts of the world, that’s as good as it gets: life expectancy for women in Botswana is only 38. And if you look closely at your chances of living to 100, and find that there isn’t much in your favour, how do you deal with that?

Even so, despite a superstitious reluctance to think too hard about the future and how or when I might die, I started looking at the things that were in my favour: a high birth-weight, a healthy childhood, good health throughout my first 50 years (not a single night spent in hospital), reasonable fitness, good nutrition, very little exposure to infectious disease, a “normal” BMI. I have never smoked or taken recreational drugs, haven’t drunk enough alcohol to damage my liver or other body parts. There is no significant history of coronary heart disease in my family. In the year I turned 50, at a doctor’s appointment (I had a chest infection which needed antibiotics), my GP suggested I have a cholesterol test, a “health check”. When the results came through he showed them to me on his computer screen and explained that the scores were remarkably low. He said, “You’re going to live to 100” (to which I said, “Can I have that in writing please?”). It was one of the things that made me feel different about my 50th birthday: if the doctor was right, my 50th birthday (rather than my 40th birthday) might be the halfway point of my life.

I made lots of small changes that year too: I started eating a bit less, cut down on red meat and processed meat, exercised more, and by the time my birthday came round I was lighter and thinner than I had been for nearly 20 years. (And that winter, having lost that weight, I felt the cold more strongly than I had for many years, and had to wear my wedding ring on my right hand – it kept slipping off the third finger of my left hand.)

I also avoid dangerous activities which other people regularly enjoy –skiing, snowboarding, anything that involves travelling quickly through water, cycling in central London. I was very saddened by the death of Natasha Richardson in 2009 (she bumped her head during a supervised skiing lesson and never recovered). It didn’t put me off skiing – I had no intention of ever hitting the slopes in the first place – but now instead of explaining the many, many reasons not to go I can summarize my aversion in five words: “Natasha Richardson and Michael Schumacher”. I am always saddened by news of cyclists being killed on the roads of central London, and each one confirms that this is one cause of death I am likely to avoid.

As earlier pieces like Oily Fish and January’s Earworm suggest, I do think often about why some of us make it and some of us don’t, about the things we can change and the things that we can’t. I don’t think about it too hard though: I still have a superstitious reluctance to think too hard about the future, or indeed write about it.