When I was 18, I dated a guy who was 29. It worked out well, and he was more thoughtful than a lot of the guys closer to my own age. Plus, he had a good job, and could afford to spoil me.
It didn't last long, though.
As everyone else has mentioned, the older guy thing can lead to problems. Mainly the problem that they're ready to settled down when you're just hitting your stride, so to speak.
Also, I've noticed it's become harder and harder to find guys around that age who haven't been divorced or already have children. Not that that's necessarily a bad thing, but at my age (I'm 20 now) I don't want to have to deal with being a "step mother" type figure, or deal with an angry ex- wife.
That's just my thoughts on it. If you can make it work for you, then more power to you.