I wonder if being done having kids affects your reactions as well. I'm 33 and pretty sure we are done with the kids now that we have the two. Maybe once you've permanently dealt with the issue of getting pregnant (one way or another), you are able to really relax and enjoy the moment. I know that even when I was on the pill and crazy about taking it at the same time every day, there was always a niggle in the back of my mind that I could still get pregnant.
I do think that being really comfortable with yourself and your partner just increases the odds of having really good sex. I know our sex life has just gotten better over time.
I think the peak thing quite frankly, is BS. I think it has more to do with how many insecurities a girl grew up with, and how long it takes to get rid of them. I suspect that for a lot of women, we don't get really comfortable with ourselves until later in life. I wasn't exactly the most positive and secure teen or young adult, that's for sure. My confidence with myself and with my partner didn't happen until much later.