'How long before people start calling you names?' ask posters launched last week in a new campaign. And, in case this is not enough to remind you to be kindlier about those we are now apparently to call third- and fourth-agers, an accompanying leaflet warns: 'At some point in your life you will find that people start to characterise you by one single dismissive criterion. Your age. Your abilities won't matter. Years of experience will count for nothing. Your achievements won't matter. Your personality won't matter.'
If this dismal state of affairs still seems distant, think again. You may even now be facing the onset of an irrelevant personality. In these recessionary times, it is harder than ever for over-40s to get a job. Early retirement is widely on offer at 50. Around half of all job advertisements quote an age, according to recent surveys: and the upper limit most often given is 35. Old is getting younger.
As careers are compressed, peaking - that dreadful point at which codgerdom beckons - happens ever earlier. Consider, for example, snooker, a sport in which you need plenty of immature enthusiasm to sustain the incredible boredom of practising. In 1981, a 12-year-old called Stephen Hendry achieved an astonishing century break on a full-sized snooker table. Twelve years later, dozens of pre-teenagers are hitting centuries every week, and a 17-year-old recently became the youngest player to reach the last 16 of a world-ranking tournament.
It may happen later in more sober, desk-bound professions, but only fractionally. According to Anita Higginson of the Brook Street Bureau employment agency: 'Perceptions of the right age for jobs are dyed-in-the-wool - 25 to 30 for the perfect secretary, under-35 for front-office staff, over-50 for cleaners. The vast majority of employers are looking to fill their vacancies with under-25s, which is crazy. People are being invited to retire early at 50 to 55, and that's when they think of ageism beginning. But frighteningly, it starts at 25 to 35.'
All of which means that by 40 in the City, advertising or television, you are probably looking elsewhere. Foreign-exchange traders burn out incredibly fast: 'I don't think you could survive in the inter-bank market much beyond the age of 35,' says Julian Simmonds, managing director of foreign exchange and money markets at Citibank. And if you can edit Panorama in your mid-30s, command a regiment in your late 30s or be a director of an advertising agency in your late 20s, what do you do at 50?
It is unclear quite why getting on in years has led to not getting on at work. Employers usually offer vague explanations: 'A young company employing young workers' or 'dynamism, commitment and flexibility'.
Yet a Price Waterhouse survey comparing under-40s with over-40s found little difference in reasoning and numerical tests; and the over-40s were stronger in persuasiveness, friendliness and astuteness, and were more competitive.
Ageism is by no means the preserve of heedless youth. Sir Michael Checkland, then 56, caused something of a furore last year when he said Sir Marmaduke Hussey was too old to remain Chairman of the BBC Governors until 1996, when he will be 73. Come to think of it, the BBC's board of governors as a whole was too old, Sir Michael said: his audience of senior ITV and BBC broadcasters broke into applause.
Sir Marmaduke has actually done very well: if he had been a rock star he could have been over the hill practically as soon as he was out of puberty. Whatever, for example, happened to Bros?
Sir Marmaduke was probably also lucky not to have been a woman. A Brook Street Bureau survey found that two people in five believed a woman with the same skills as a male was more likely to be discriminated against on grounds of age. Women television presenters are not allowed to grow craggy and authoritative: from Angela Rippon through to Zeinab Badawi they all have to be better-looking than their male colleagues.
Peter Naylor, author of Age No Barrier, believes that 'age is a crap criterion to use' and thinks our current obsession with it is a legacy of the Sixties. Michael Young, a social scientist, points out that birthdays have become public events, which we must all declare in public to show we're entitled to go to school, have sexual intercourse, buy alcohol, vote, and pretty much everything else.
He would like to see age come within the scope of an extended Data Protection Act, as information which it would be illegal to solicit, or to use to discriminate.
Dr Eric Midwinter, author of a 1991 report on attitudes to ageing, believes one of the problems is that 'old' is the opposite of 'new', as well as of 'young', and so the term is 'overlaid with grey colourings'.
But for whatever reasons, assumptions about the failing powers of codgers are widespread. 'People don't usually improve with age,' says Sir Kingsley Amis, 70. 'Novelists are no exception. On the whole, it's the earlier works that people are remembered for.'
Meanwhile, in every field, you can identify people who were past it when still youthful - either because they took themselves too seriously (pop stars such as John Lennon or Sting); or because they burned out (Andrea Jaeger, the tennis player); or simply hit a bad patch (Lord Moore of Lower Marsh: as John Moore, a Cabinet minister and tipped to be Mrs Thatcher's successor, but now, at 55, in the relative obscurity of the Upper House).
But in every field it is also possible to identify ancients who are still on top: Cyrus Vance is 75; Marje Proops, coy about her age but either 81 or 82, has just ousted a younger woman from her job; Mary Wesley is still turning out sexy bestsellers at 80; Jack Jones is 79 and militant about pensioners' rights. The trick to keeping your personality relevant and having no one call you an old moo is, apparently, finding a new hill as soon as you get to the summit of the first one. Sir John Harvey- Jones, still going strong at 68, prides himself on taking some sort of course every year.
And John Major, 49, told Jilly Cooper in Hello]: 'I'm going to try as hard as I can. But I'm very young, and if it goes wrong, I can always do something else.'
Additional reporting by Hester Matthewman