What is it that is so horrifying about the ageing rock star who just won't hang up his guitar and accept that the final curtain ought to have come down? We don't object to actors coming out of retirement for yet another final performance. Perhaps it is because the rock star is an emblem of youth and sex appeal, so when they return, looking like wrecked pastiches of their former selves, it sends a shiver up our spines, reminding us that we're mortal too.
Axl Rose's disastrous turn at Reading Festival was a striking example of the genre. Looking bloated, botoxed and breathless, the performance of the Guns N' Roses star – who disappeared for many years to work on an album – collapsed into mayhem as disappointed fans booed and he swore back at them through a megaphone. It was almost reminiscent of one of Judy Garland's memorably awful "final" tours, when she would fall asleep in a song and tumble off the stage.
Not all musical revivals are as embarrassing. But the key is: if you were young and pretty then, you'd better look pretty fit now. This is the secret of Take That's comeback, which would have flopped had they all been sporting beer guts. Generally, however, if you've been away for a while, it's best not to come back.