The Inbetweeners: Has anyone seen my left testicle? Yeah, only 15 times

The Inbetweeners, E4, Monday, and The Young Ones, BBC1, Tues-Thurs

Has The Inbetweeners gone off the boil? (Certain TV reviewers would have asked if the show had "jumped the shark", in reference to the moment Happy Days began its slow decline; but "jumping the shark" sounds like the sort of puerile euphemism, of which The Inbetweeners show is fond, for some icky sexual practice.)

I ask tentatively because I like it, this joyously filthy sitcom about the failure of sixth formers Will, Simon, Jay and Neil to keep their hormones under control. And, according to the viewing figures for last Monday's episode, the first of its third series, so do 2.25 million others.

But when we had been treated to the 15th shot of Simon's left testicle – yes, his actual testicle, yes, I counted the shots – as, unaware, he paraded up and down the catwalk of his school fashion show, you did wonder where the writers, Iain Morris and Damon Beesley, are going to take us from here.

We'll see. Until then, this unholy union of American gross-out movie, Carry On-era comedy of embarrassment and Roger's Profanisaurus is still an unrivalled primer for the sordid inner life of the Middle England schoolboy. If there is a problem, it's when the show steps out of its (dis)comfort zone. The vehicle for the testicle joke was the school charity fashion show on behalf of another pupil with kidney failure confined to a wheelchair – which gave the writers a chance to have a dig at the deserving sick kid who somehow gets on everyone's nerves. Naturally, he's a boy – which does make you think, why not a girl? Now, I've only watched a few episodes of The Inbetweeners, so I'm happy to be corrected, but girls never quite seem to be accorded enough attention to be the butt of a gag. Or maybe I just don't get enough "clunge".

Half an hour in the company of The Inbetweeners is enough to remind most of us why the passage of youth is not entirely to be mourned. Indeed, some may not mind the idea of an old age spent dribbling in front of Jeremy Kyle. For those of us who do, The Young Ones was required viewing.

Half-a-dozen worn-out celebrities, with barely a functioning limb between them, were coralled into a house and told that after one week of pushing themselves physically, declining offers of assistance and enduring Lionel Blair's Sammy Davis Junior anecdotes, they would be better off all round.

So far, so obvious. But it was the premise of the programme's resident boffin, Ellen Langer, a twinkly American academic, that returning the subjects to the physical environment of their prime would benefit them even more. Which is how we came to be watching seventy- and eightysomethings Sylvia Syms, Dickie Bird, Liz Smith, Kenneth Kendall, Derek Jameson and Lionel all cooped up in a 1975-vintage house, struggling with its eye-level grill and wincing at the tiger-print sofas.

Whatever the gerontological importance of a strategic bit of shagpile, the décor was at least a distraction, as it took about an episode and a half for Derek to climb the stairs (there were three episodes in all, scheduled over consecutive nights). And it was certainly no surprise to see Sylvia light up when she was given half-a-dozen kids to look after, or Lionel showing he's still got it choreographing a chorus line of blonde lovelies.

What was undeniably affecting, though, was the simple joy each of them took in being considered useful once more. Backs straightened, gaits steadied, smiles returned. Which in the case of the delightful Liz Smith, 88, was a very big smile indeed.