The other day, I gave a talk to a group of investment bankers. I did what I always do when I'm in front of a business audience: scan the crowd and try to work out how many men there are to every woman. If they are young City of London lawyers, the numbers are usually roughly even, while for senior bankers and financial advisers, it can be as bad as one to 20.
On this particular afternoon, the ratio was a bit better than usual - about 1:4 - but as I looked around, it occurred to me that I was counting the wrong thing. The tiniest minority was not women. It was not even people from ethnic minorities, this being a global conference. It was the over-50s.
In about 200 bankers, I could see only one person who seemed to be my age - and that was the chief executive. As I walked back through the City, I stared at the people going home: a sea of commuters in their 20s, 30s and 40s. Only occasionally did I spot a contemporary, sidling past with head down. I briefly got excited when I saw two people who looked about 60, but on closer inspection, their brightly coloured anoraks and the suitcases they were wheeling revealed them to be tourists.
The disappearance of 50somethings from offices in London may not be new, but I have been slow to notice it. That's probably because it is still possible to be over 55 and a journalist on The Financial Times without feeling too outlandish. It is hard to feel too exposed when the finest and most valuable columnist on the FT has a good 10 years on you.
The same is not true in other parts of our business. Recently, there was a fire alarm in the office and I looked at the human snake of colleagues from the commercial departments shuffling down the stairs. Number of people my age: zero.
Possibly it is just a case of the policemen getting younger, but I don't think so. A couple of fellow journalists in their 50s assure me that they are the oldest people on their commuter trains from St Albans and Muswell Hill arriving in the City every morning.
The tiniest minority was not women. It was not even people from ethnic minorities, this being a global conference. It was the over-50s. In about 200 bankers, I could see only one person who seemed to be my age - and that was the chief executive.
A friend who is about to turn 50 in a large consumer products company is keeping quiet about his age and hoping that no one notices him. When he joined 20 years ago, there were plenty of people in their late 50s, often with a PA of the same age or older. Now there are no more PAs of any age and the managers mostly slope off in their late 40s, having been handed a fat cheque to do so.
The few who hang on in mainstream corporate jobs fall into two tiny camps: the highest flyers, who are either a chief executive or hoping to become one; and the lowest flyers, who have succeeded in making themselves invisible and avoided all rounds of redundancy.
This elimination of the vast rump of 50somethings from London's office spaces is at odds with what is supposed to be happening, which is that people are working longer, not just to a normal retirement age but beyond. In the past 10 years or so, the stats show, the number of people in the UK working beyond 64 years of age has doubled .
If investment bankers, lawyers and accountants are an exception, that is neither puzzling nor worrying. By their late 40s, they have earned so much money to have no need for more, and after 25 years of working all hours in dysfunctional environments, they have generally had enough. They are less victims of ageism than a product of how the system works. They have done the hard graft and can now do something more pleasant with their time - either nothing at all, or a bit of consultancy, or be born again as a photographer or landscape gardener.
But for the next slice down, it is utterly baffling. Where are all the 50somethings who used to do standard jobs in human resources, or marketing, or events? Who employs them? Did they receive generous enough redundancy payments - and have enough money in property - to get by earning a bit extra here and there?
Whatever they are up to, the pattern must be about to go into reverse, for reasons we all know: pensions are worse, health is better. If we live to 100 and have to work until we are 75 to support ourselves, big companies will have to start taking us back. Fifty- and 60somethings will cling to their office jobs for dear life or, if they lose one, will go searching for another - possibly on a lower wage - so that employers will have to stop thinking of ageist excuses not to hire them.
HR departments of large employers will soon laugh at the fuss they have made about the non-problem of how to keep spoilt millennials happy.
Motivating 50somethings who have already tired of the nonsense of corporate life but still have to slog on for another 15 years?
That is going to be the hardest management task yet invented.
THE FINANCIAL TIMES