The age of rock, or ageing rockers?

Bono's back injury caused U2 to pull out of the headlining slot at Glastonbury this month.

It was the kind of statement one might expect from the spokesman for a stricken footballer whose dreams of World Cup glory have been dashed by injury. "He really wanted to be there to do something really special. He does feel terribly that he has let people down. He loves performing. He has not only just got out of hospital, he is not in a good way. At the moment we are all just very upset." But these shellshocked words weren't uttered by Cristiano Ronaldo's, Wayne Rooney's or Lionel Messi's "people". They came from the various talking heads in and around the U2 camp when it was announced the Irish band was pulling out of its headlining slot at Glastonbury this month due to Bono's back injury.

It was all very sad - although the subsequent announcement that Gorillaz would replace them sugared the pill somewhat. But the real subtext was clear. Bad backs are, in the main, the preserve of the old. Let's face it: there's little less edifying a prospect than watching the heroes of our youth trotting out the same old moves 30 or 40 years on - not least because it reminds us of our own mortality. Anyone who thinks the Rolling Stones have still "got it" because they're once again at No 1 in the charts should remember that the record outselling Keane and Faithless, Exile on Main Street, is 40 years old this year. Would an album of all new Rolling Stones songs do half as well? We very much doubt it.

That the fire has never gone out from Mick Jagger and company is not in doubt. But whether that means the rest of us should have to be impressed is a moot point. Take, for example, the news that the Rolling Stones guitarist Ronnie Wood is keen to reform the notorious 1960s bad boys the Faces for live shows. A reunion might have been interesting: Rod Stewart could return to the days when he was a throaty frontman rather than a smooth crooner. But he wasn't interested, so whom did Wood turn to? Of all the vocalists in the wide world of rock, he asked Simply Red's Mick Hucknall.

Sure, Hucknall might boast to all and sundry that he was at the infamous 1976 Sex Pistols gig in Manchester that kick-started punk, but a less rock 'n' roll frontman you couldn't hope to find. Perhaps money is indeed too tight to mention for all concerned. Still, he's younger than Bono. Just. Rock stars enduring well past their sell-by date shouldn't be so much of a surprise: the nostalgia market is a lucrative beast, after all. Those who have survived the stumble into old age with reputations intact, though, are unquestionably the artists who haven't been afraid to diversify.

Bob Dylan's best work of the past decade hasn't been his music at all but his radio show, Theme Time Radio Hour. Neil Young has been as happy to play stripped-down sets in theatres as to rock out Glastonbury with his band - and was all the better for it. Kraftwerk may have been in cold storage, but when the pioneers of electronic music came back as sixty-somethings, Ralf Hutter played the song Tour de France live in a velodrome with the UK cycling team circling around the audience. The encore was in 3D - and how many gigs can you say that about?

So our proposed upper age limit for rock 'n' roll (let's say 50) comes with a caveat: anyone born before 1950 can of course continue to be involved in the music business, but only if their project is sufficiently innovative or interesting. Gil Scott-Heron, the 61-year-old famous in the 1970s for jazz-funk and soul, is allowed to continue thanks to the brilliance of I'm New Here, a fantastic new album melding electro, folk, hip-hop and dubstep. But sorry, Van Morrison, you're out. One farewell tour for anyone left on the planet who hasn't yet seen you, and that's it.

We're joking of course: the notion of pop music being solely a young person's game, full of squealing Lady Gaga clones or preppy bands in thrall to Vampire Weekend is almost as frightening a prospect as the Rolling Stones touring in their seventies (less than two years away for Charlie Watts, lest we forget). But next time you see an underwhelming performance by a rock "legend", and only give their new album a cursory listen, think on this: are you investing in their continuing career because you really want to, or because you think you should?