Most Helpful Guy
I've almost always done the inverse; being involved with women even more significantly older than I am. Generally speaking, I find older ladies and older men easier to talk to and be friends with than those younger. Though I'm not typical, I'm pretty sure that I'm not pathological. :)
Which means that there is a similarly legitimate reason to prefer those younger. That said, it is usually far easier to emotionally manipulate younger women than older ones simply because the older ones have the benefit of experience and professional stability that is impossible for someone in their late teens/twenties.
If the deal is consensual, casual sex without any sort of emotional arm twisting, cool. Everyone should be able to have fun with whomever they want, however they want. Manipulation and/or abuse are a different story, but don't jump to that conclusion without real evidence.
I'd happily hook up with a freshman if I ever came across one that I found really interesting and pleasant to converse with. That seems quite unlikely, though. When I took a course at the local community college a few years ago, I ended up sitting with and working on projects with a group of real 18-19 yo freshman.
They were quite cute and their dedication was honestly impressive, far beyond what I displayed at that age. But they dynamic was one where I was more of a mentor than a peer. Indeed, the only person I ever spent time with outside of school was the professor, whose family ended up being from the same city in India as my dad. That, and he was a political junkie as I am.
Most Helpful Girl
A lot of guys who only want sex will go after younger, more naive women because it's easier to convince them to do stuff a lot of grown women won't do with a casual partner. They're also easier to talk into sex in general.. easily duped..