Age has very little to do with it, IMO. Whether you're 21, 41, or 61, if YOU feel you're not having the sexual libido that you should then that's more than enough reason to bring it up to a Dr. When I was 21 myself, my Dr inadvertently switched my birth control pill to something that was supposed to be "same thing, different pkg". It wasn't. It took me 8 mos of a disappearing libido to figure out something might be wrong. When I mentioned it to him, he said, "Oh, yeah, that pill DOES get that complaint a bit..." (?!?!?!?!)
Another thing to consider regarding the whole "sexual peak" for women: yes, part of it is due to biological clocks and such, but a lot of new studies have suggested that it often takes women UNTIL they're in their late 20s and early 30s to figure out what works for them sexually (by nature and by nurture -- i.e. societal influences -- we're not the easiest creatures to get to comfortably climax) ... and when they do, watch out!! LOL! 
It's positive to note that you recognize SOMEthing is different that allows you enjoy sex more with your fiancee. Pay attention to whatever those cues are and go with it. Also, you may want to do a little investigatory research into a certain buzzing & humming little toy... If you've never really done much "exploring" on your own, that could really help you to figure out certain things you like, and rather quickly too! Most importantly, keep the dialogue with your FH open. Many women do NOT climax from just sex; they enjoy it, yes, but you're right that it often takes additional foreplay or other things to get them there. Try to be "seductively" vocal when he does something right; it's one of the fastest ways to get him to do it again! 
So talk to your gyno too! 
