I've heard plenty in the media recently about the comedy industry still being riddled with sexism, but I've heard less about the industry being riddled with ageism. Well, not much anyway. I was watching Alexei Sayle on Breakfast TV this morning and the sixtyish stand-up was saying that one of the most gratifying aspects of his comeback shows is the fact that there are so many young people in his audiences.

But it is relatively easy if you are the Godfather of Alternative Comedy like Sayle. It is less easy if you haven't made it anywhere near national treasure status. Whenever I write something about comedy on Facebook there is always one comedian - you know who you are - who brings up the fact that one of the things that has held back his TV career is the fact that he is the wrong side of forty. Stewart Lee had to fight hard to get a few older comedians on his Alternative Comedy Experience. I suspect he would have liked to have featured more but Comedy Central probably thinks its audience wants to see comedians who resemble the cast of The Big Bang Theory, not the cast of Last of the Summer Wine.

When I wrote a piece yesterday about there being no such thing as an overnight success, someone else remarked that while it certainly helped you to get better if you put in the hours, television, particularly digital channels such as BBC3, is always more inclined to pluck a new face that fits what it thinks its audience wants, and that face doesn't usually have any wrinkles on it. I don't think this is always true - Nick Helm, for instance, is no fresh-faced youth and TV is currently courting him and Greg Davies is well ancient but seems to be getting plenty of work – but maybe it is a tendency. And maybe it has gone on for a while. Arthur Smith has occasionally mentioned the fact that he is more Benjamin Button at the start of the Brad Pitt movie than at the end and this might have held back his TV career when he was younger.

Comedy is always being compared to rock and roll but it is actually more like pop music. Comedians come into fashion and then go out of fashion. This had never occurred to me until a few years ago when I realised that Harry Enfield, who had been at the top of the tree at one point, was not on television as much any more. The other night he was back on television – playing Jack Whitehall's dad in Bad Education, which underlines the fact that while he can still be as funny as ever when he gets the chance, he is now part of a previous comedy generation.

While television looks for fresh blood as eagerly as a thirsty vampire, newspapers are not much better. They would often rather hear about someone new than someone doing good work has has been around for a while. But whether that someone new has the legs under those skinny jeans to still be attracting young crowds when they are Alexei Sayle's age is the real test of a true comedian..