I saw someone as young as 32 writing her memoirs, and she lived until her 60s, so it's not an intimation of impending doom. Here in the UK some 'celebrities' such as footballers or models will often publish an autobiography in their early twenties if they think they can make some money out of it. They'll use a ghost-writer, though, and probably can't put a comprehensible paragraph down on paper by themselves (cynical, moi?).

Now that you mention it, I've mostly seen women writing their memoirs, but a couple have been men.