One thought that occurs to me reading the Vintage Classics edition of Morvern Callar: is twenty years too soon to call a book a classic? (Granted, in this case ‘classic’ is more a marketing term than a critical evaluation, but the question remains.)
I don’t have a straightforward answer. On the one hand, my instinct is to say that the term ‘classic’ implies a book old enough to have survived being tested by the years, and twenty years is not old in literary terms. On the other hand, I very much liked Morvern Callar, and wouldn’t hesitate to recommend it. If a ‘classics’ imprint can look at a certain period in time and say, ‘these books are worth reading now’, I find it hard to begrudge that.