Disconcerting news for Last of the Summer Wine fans, whoever they might be. So anxious are BBC producers about the frailty of their octagenarian actors that one, Frank Thornton, 87, has been banned from filming outdoor scenes for the new series. For insurance purposes he must be filmed in the safety of a studio, presumably with a paramedic, a wet flannel and some Werther's Originals to hand.

I cannot pretend to care about the verisimilitude of LOTSW, finding it about as funny as a feminine itch. But it does seem a bit rum to assume that elderly thespians will drop dead if they so much as set foot in a field when half of Hollywood's young actors are busy mainlining Temazepam and Red Bull while stuffing their anorexic stomachs with kitchen roll.

But then ageism is the only “ism” that one can brazenly get away with these days. You cannot satirise someone's race, gender or disability - but a codger? Fill your boots. I'll admit to ageist moments myself, such as when I'm tutting in the queue for the cash machine and the elderly person at the front is bent double over the keyboard slowly pressing every single option key and always, always requests a receipt. Or driving in the middle lane of the motorway, invariably wearing a flat cap, at 25mph.

No, it isn't funny to mock the aged. Except that sometimes it is - as American comedians gorging themselves on the hilarity of 71-year-old John McCain's decrepitude have discovered. It would take a heart of stone not to laugh at some of the gags: John McCain looks like a Wal-Mart greeter/ like the guy who brags that oatmeal has lowered his cholesterol/ who picks up his TV remote when the phone rings.

David Letterman excels in this field: “John McCain looks like a guy whose head you can barely see over the steering wheel... who thinks the nurses are stealing his stuff... like the guy at the supermarket who is confused by the automatic doors.” Someone has even posted a website: how to tell John McCain jokes (basically just mention Viagra/ hearing aids/ dentures and you're off).

Yes, faced with the minefield of making fun of an African-American and a woman, most commentators have played safe and stuck it to the fogey. As The New York Times said: “It is safe to say that not since 1996... have [old] geezer jokes found such a cosy home in presidential politics.”

But why? It's not as though, as with race or gender, a person can do anything about being old, except perhaps to die (thus, I suppose, shortening the queues at the post office). Why does old age confer a licence to disparage?

Perhaps, deep down, the younger generation envy the elderly their freedom, their early-bird specials and the fact that if they break wind in public people understand.

And, actually, at least Senator McCain can congratulate himself for still being a player. If he was a woman aged 71 he wouldn't stand a chance.

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