Razib Khan distills a 2010 study of American sexual behavior:
Before the age of 25 it seems that women are more likely to have sex in a given year than an equivalent age man. After the age of 25 this starts to reverse, and men are more likely to be having sexual intercourse in a given year.
In the comments section, Khan adds:
I put this post up because it seems to quantify what I’ve heard of/seen anecdotally. e.g., I have attractive (young looking) female friends in their mid-30s who are shocked all of a sudden how difficult it is to date because now they’re competing with women 10 years younger. To make things work they have to move up themselves to 45 year olds, and they’re not always willing to do so…. The converse is that to some extent immature and broke 20 year old males compete for the same women with 25 and 30 year olds who have jobs and are more seasoned.