When I was 24 I had a girlfriend who was 39. It was fun for a while: she wanted sex, she wanted it often, and her biological clock had already gotten disconnected so she wasn't pestering me about "where is this relationship going?" -- we were there to have some fun for a while.
At the time, the women I knew in the 20-24 age bracket were looking for guys older and more financially secure than me, so it was a good change of pace