When I was 21, I got in tow with a 33-year-old woman and had a great time for a couple of years.
I think some of the attraction from the woman's point of view is that a) it's very flattering to be chased by a younger guy, b) he's usually got more stamina than a guy her own age and a much shorter recovery period, and c) he probably hasn't finished working out how he likes to do things and is more willing to experiment. A lot of guys in their 30s and 40s know what gets them off quickly and then they roll over and say, "Was that good for you too?" Can you blame a woman who's been on the receiving end of that kind of attitude for taking on a younger guy who'll ask the same question after the fourth or fifth round instead of the first?
That was certainly the case with me and my ex. She'd been with an older guy for a few years. As far as I knew, she lost her virginity to him and the sex was pretty much the same from Day 1. I didn't have much experience when we met, but we sure learnt a lot together.