I exaggerate slightly. Babies I can of course pick out, and small boys wearing Arsenal and Manchester United football shirts (though even then you have to be careful you're not confusing them with senior management in BBC television), but otherwise I am at a loss. If somebody were to tell me that there are no such people as under-25s, that a very precise age-sensitive epidemic wiped them all out while I was chair-dancing with the over-50s in Eastbourne, I can't see how it would make any significant difference to my visual universe. I never noticed them when they were there. To the elderly the world looks elderly, and if the prospect of that doesn't make you wish you were old already then you lack imagination.
When I do, as in my profession I occasionally must, keep company with fellow writers in their thirties - mere beginners if you want to know the truth of it - I am startled by the gap not only in our culinary and tailoring aesthetic, but in our appreciation of what makes for beauty in the other sex.
They gasp and nudge me when a squidgy weanling who is all bulge and baby- fat above and all grey bone and angularity below trips by on built-up Reeboks, and I gasp and nudge them when a panting grandmother experientially lined at the corners of her eyes, exquisitely wonky round the jaw, pauses at a lamp-post to catch her breath, repair her lipstick and take the weight off her toeless two-toned Charles Jourdan stilettos.
"Now tell me," I whisper, when I get my own breath back, "that any woman not past the age of child-bearing will ever look beautiful to you again!" And they stare at me as though my brain's gone soft. But I mean every word I say. To the elderly the world is elderly, and only the elderly can steal your heart.
Is there a democracy in this? Did we feel similarly when we were young, that the only world that mattered was not a day older than we were and that the rest of it could go hang? Not me. What I remember most about my youth was that I wore it like a chain around my neck. Being the age I was - I'm talking roughly seven to seventeen now - embarrassed me acutely, as though it were a guilty secret, a moral rather than a physical condition which I would have hidden had I known where to hide it.
Immaturity plagued me; it felt like a permanent finger of shame pointed at my intelligence. I routinely fell spiritually in love with older men in whom I saw the self I couldn't wait to be, but in whom I also saw supreme contempt for my inexperience. It came as a nasty shock to me, years later, to discover that older men themselves fall in love with younger, loving them precisely for that quality of unusedness which I despised in myself. It won't come as a nasty shock to you, then, to learn that I find such a passion inexplicable, except as a morbid perversion, and of course I don't have any objections to those.
It's exam results week which has turned my mind to this. All those letters flying from post office to post office, all that hope hanging on the whim of a postman. What if he happens not to like you and decides to feed your four A-pluses to the dog? What if he chooses to swap envelopes just for the hell of it? Sure, you can pop round to your school, if that's what they still call it, to check your results on the noticeboard, and no doubt you can ring someone up and complain if you don't like what you see. But those are hurdles two and three, and you won't be in any condition to clear them if the postman has already brought you down at hurdle one.
I am, of course, remembering my own fears. Good exam results were the quickest exit from the ignominies of youth. Get 20 O-levels, each a more spectacular A than the last, and no one would ever mistake you for a young person again. More to the point, get 20 O-levels and the road to blessed adulthood stretched beckoningly before you - sixth-form, university, and then LIFE! Fail and you'd be a child for ever.
So you couldn't afford for there to be any mistakes on the morning the postman was meant to call. And yet you only had to think about it for a second to imagine the thousands of mishaps that still stood between you and maturity.
Postmen get tired, postmen have heart attacks, postmen suffer sudden and irrecoverable memory loss when it comes to streets and house numbers. All it needed was for the postman to lose his footing over a grate at the very moment he was separating your letter from the bundle and that was your future down the drain.
I wouldn't wish such a fate on any young person, whether I know what one looks like or not. Get that letter, get those passes, and get out of juvenescence. Trust me, it's much better where I am.
Howard Jacobson's new novel, `The Might Walzer', is reviewed on page 9