There is this myth or very old data that for women it is in their 30s and for men it is when they are 18. It was discovered in the 1950s by sex researcher Kinsey. Back then most women did not have sex before marriage, didn't masturbate, and were naive about sex, and men had much more experience, so when they did the survey in the 1950s, women weren't very experienced. Today, both men and women have knowledge about sex spread throughout their adulthood. There isn't a sexual peak per say.
Women can have two peaks.
They can peak before they have kids and after they have kids. It is hard to peak with kids. As a spouse, part of your job is to try to get stress out of her life. You have to do something special for her. My wife is 42.