Ouch! That was probably the reaction of a few people to the news that Tim Mason, the 50-year-old head of Tesco's US operations, is ruling himself out of the race to succeed Terry Leahy as the company's next chief executive on the grounds that he is too old. It was mine.

Anyone who is 50, or approaching that age (for those wishing to send me a card, May 31st, 2009 is the date), must feel a pang at the idea that we are over the hill.

Mason's logic is that Leahy will be there for some years yet and the board will seek a successor who can stick around for a bit.

To judge by the average age of chief executives in the US, Mason may be being a little over-sensitive.

According to Spencer Stuart, the head-hunting firm, the median age of new chief executives of S&P 500 companies last year was 52.

Sprint Nextel recently appointed Dan Hesse, a "54-year-old telecom veteran", as Dow Jones put it, to be its new chief executive.

His sentiments certainly fly in the face of the campaigns encouraging us to respond to the ageing of the baby boomers by rethinking our bias towards youth, both inside the workplace and outside.

"My feeling is that people have to change their perception and say: 'I am still going for it. I am a contender'," says Jeri Sedlar, a senior adviser to the Conference Board, the US business research group, on such matters.

But UK companies appear to favour fortysomethings these days, perhaps encouraged by the practice of splitting the jobs of chairman and chief executive, which allows experience and energy to be bundled separately.

Personally, although chief executives, like policemen, look younger all the time, I am less taken aback.

A 50-year-old is not yet over the hill in most companies but he or she is at the brow. The median age of chief executives has fallen steadily, albeit slowly, for the past quarter of a century. I do not see the harm in that.

It is true a board would be stupid to rule out anyone over 50 for the top job. It was not so long ago that most US chief executives were in their 60s and that was in the days when more of them drank and fewer took exercise.

Looking at the rude health of those such as Rupert Murdoch, who is now 76, there is no reason to doubt the stamina of many 60-year-olds.

It would also be counter-productive in demographic terms for companies to be too youth-oriented.

McKinsey estimates that US companies will need to fill 34 million net jobs between 2005 and 2015 as baby boomers retire.

There may not be enough people in their 40s around in western societies in a few years to rule out the over-50s.

That said, the notion that the median age of a new chief executive at a big US public company will settle at 50, with lots holding the job between their mid-40s and mid-50s, strikes me as reasonable enough.

One reason that chief executives used to be older is that more stayed at the same company for most of their careers and gradually worked their way up.

The corporate world was also a more stable place with less job-hopping.In that world, it made sense for jobs to be arranged according to an age pyramid, with people hitting the top within striking distance of retirement.

Having a younger chief executive would have disturbed the carefully constructed system of training and advancement.

But it is fading and an open market for executive talent is emerging instead.

The average tenure of chief executives is falling as investors become less tolerant and there is greater demand for executives in their 40s with leadership potential.

For the most part, these recruiters are acting rationally. They know that someone who has run a couple of divisions at a big company by the time he or she is 45 has enough experience to be given a shot at a top job. There is no need to hang out for even more grey hair.

This sounds depressing for the over-50s but it need not be. Not everyone, after all, wants to be a chief executive. It may be well-paid but it is also a highly demanding job.

"There is such a focus on performance and everything is instantaneous," says Gary Burnison, the 46-year-old chief executive of Korn/Ferry, the head-hunter. "I shut off my cellphone and my BlackBerry at 10pm."

There are plenty of other things to do. In demographic terms, the next few decades will be a good time to be an older employee since there is likely to be a shortage of trained and experienced professionals.

More companies are also organising themselves like Google, video game studios or drugs laboratories, with a lot of skilled employees being given autonomy rather than everyone working in a strict hierarchy.

That places a premium on talent and enterprise rather than age and willingness to conform.

As for Mason, he may not be Tesco's next boss, but he has sent a signal that he is available.

They're wily, those 50-year-olds.