I don't think this is just about age but lifestyle. When I was an undergrad I could easily sleep 12 hours a night, and when I graduated 8-9 hours easily. But now, being a working person, I generally get about 6-7 hours sleep every night.
However, it's dawned on me recently that this is largely because of work commitments, deciding to stay up a little later that I should, having more responsibilities that make me think more and occupy my mind. So for ages I thought "I am a little older so I need less sleep" but I don't think this is the whole case.
I've been on leave in the last week and every night I've slept between 8-10 hours, which is normally unheard of for me. And that's because I've been eating dinner a little earlier, going to bed earlier and getting up later because I don't have to go into work. So I think lifestyle plays more of a part than we realise, we just get used to things and don't realise that the small changes we integrate can make big differences.