I've read studies it's in the mid 30s or early 40s and some women even claim they believe they hit it after menopause. Ultimately every woman is unique and different. Some women have never cared for sex and see it as a chore.
From personal experience, I'd say the mid-30s to mid-40s is right. A woman is less likely to have to worry about pregnancy but has the physical benefits of having gone through a pregnancy or two. (Yes, there are some benefits for sexual development.) I actually wrote a hub once on this subject, but it was vetoed by my husband for being too personal. Only hub he's ever asked me not to publish.
It all depends on the man the woman is peaking with in my opinion. My sexual peak happened when I was having sex with the one I loved. I was comfortable, felt safe and I mean hell he was pretty damn good. All depends on different factors.