They just get wrinkly and less hot. I think it's a decent enough cutoff for evaluating someone as a 'babe'. Women in their 30s are at a disadvantage compared to women in their 20s.
Looks like that Monty Python retrospective arrived just in time to crown this as the winner in the current TOTY contest.
As for the subject of the poll, yes Olivia Wilde could use a couple of pounds in strategic spots, but she's still pretty damned close to perfect. And in interviews, she seems smart and good-hearted, adding to her hotness.