Ohhhh, Jess, my fellow horny Capricorn - we really have a rough time with this issue!
First of all, I'm almost 27, and I'll tell you that if you have a high sex drive to begin with when you're younger....it's not going to "die down" with the years - so you might be at your sexual peak for a loooooong time. I wouldn't say the guys you've dated are "past their prime" because they're older - it may just be that their sex drives are not as high as yours to begin with. It can get frustrating to find a middle ground here, and my only advice is to be open with him about it. Otherwise it's going to wind up with you feeling rejected and resentful...and never being able to explain why.
I agree that it's wrong for it to turn into a situation where he can get it whenever he wants it but not the other way around.
People might suggest that you turn him down once in awhile to turn the tables on him - but who are we kidding? I have a feeling you're a little like me in that, if he's putting the moves on you (and even if you're not in the mood at the moment, putting the moves on can easily change your mind), then why the hell would you turn him down? It'd be like someone offering me a free pint of Guinness and me saying, "Yeah....no, thanks." It's a waste of a good opportunity to enjoy something you love - you just don't want to take that stuff for granted.
What I'm trying to say is, I'm not going to say "Shut him down once in awhile" like most people would. But here's some links I found that I hope will help:
Tips For Dealing With Different Sex Drives In a Relationship - Info Barrel
Tips for dealing with differing sex drives in couples - by Douglas Black - Helium
Tips for dealing with differing sex drives in couples - by Bridget Webber - Helium