Will I live longer than my cat?

David Spiedgelhalter's cat Mr Cat
Image caption This is the author's cat. This cat is not dead

As he muses about his mortality, statistician Prof David Spiegelhalter wonders if he is destined to live longer than his 20-year-old cat.

Our cat is old. Old, deaf and a bit daft. But, as I steadily head that way myself, I've started to consider him as a role model.

He's over 20, and in the recent unseasonable sunshine has taken to lying corpse-like on the pavement. In a feeble impersonation of Schrodinger's cat, he could be either alive or dead, and the only way to find out is to prod him, as he doesn't respond to shouting.

Last week, he took to doing his death act on top of a bin, and so it looked like he had just been thrown out with the rubbish. He got kidnapped by a concerned cat lover and carted off to the local Blue Cross, and we had to go and bail him out.

Taking each cat year as seven human years makes him over 140 - twice the human three-score-years-and-10 Biblical use-by date. I recently "celebrated" my 59th birthday, which is only around eight cat years and so a relative youth.

Being a statistician, I naturally wonder what proportion of my life has already flitted by, and the Office for National Statistics (ONS) life-tables tell me that, assuming things stay the same as now, an average man my age can expect to live another 23 years - that is, until 82. Still way short of the cat, and suggesting I may have already had 72% of my life. Not encouraging.

But naturally I believe I am healthier than average, just as most people think they are better than average drivers. Of course, I could be unlucky and get knocked down by a bus tomorrow, or be lucky and slog on to 100.

The stats tell me that I have a 1.4% chance of scoring a century and getting a letter from the Queen. In fact, there is at least a 6% chance of the Queen getting a letter from the Queen (or whoever has the job in 2026), but she is not in the least bit average, and has good family precedents, with her mum hitting 101.

Our survival is governed by the "force of mortality" - the wonderfully archaic expression for the chance of dying each year. Each year, an average adult ages, this unavoidable force increases by around 9%, so that every eight years your chance of not making your next birthday roughly doubles.

But as the UK has got safer and healthier, the force-of-mortality has been decreasing for decades, so that life expectancy has been rising at about three months a year - it's odd to think we have been essentially ageing only nine months for each year that passes.

The ONS also make additional projections assuming these improvements in health continue, and their central estimate gives me another 27 years on average, until 86. Amazingly, their "high" projection suggests that girls born now could on average live to 105 and boys to 103. By this type of reckoning, the future monarch will be spending their entire day scribbling letters to centenarians.

Even under more conservative assumptions, there are going to be a lot of retirees to support - if you listen carefully, you can hear sobbing in the Department for Work and Pensions. Since we already have had Jeanne Calment lasting until 122, and many people living to over 110, I would bet that someone alive now (although probably not me) will stagger on to 140 and match the cat.

All these projections are controversial. Some researchers believe that basic biology, with disease and decay coming from random cell mutations, cannot be substantially altered. They argue that past trends will not continue - we can push the envelope, but only so far and then chance inevitably takes over. Sir Richard Doll, the great cancer epidemiologist, said it was essentially a matter of luck whether you get cancer or not. But a lot of money is going into drugs to slow ageing, and so maybe the first to 140, just like some past Olympic athletes, will be drug-assisted.

Doll also came up with the "biblical use-by date" phrase to describe the 70 years of Psalm 90 (although he lived until he was 92 without going mouldy). The Psalm continues gloomily with "and if by reason of strength they be fourscore years, yet is their strength labour and sorrow; for it is soon cut off, and we fly away". If we are just adding years to life, rather than life to years, then we may not have so much to look forward to.

But I remain inspired by our cat. He hasn't exactly been lavished with affection, and has never even had a proper name, just being called Mr Cat by default. But he's kept plodding on, year after year, expressing his feelings by jamming his claws into delicate parts of my body. He fears no dog.

A perfect role model. So when I, too, am old, deaf and a bit daft, I shall follow his example by sitting outside the house, alternating between shouting at passers-by and pretending to be dead. I've got the bench set up already.