Anecdotal, I know, but 18-19 is when most people go to college, and get to shed whatever labels they had in high school. It certainly happened that way with me and my college friends.
You mean going from a pretty gender balanced high school, to a department with a large majority of men? (we were talking about nerds here, and I am talking about things like CS departments, since this is HN.)
And also to a place where no one will judge you because you're a nerd. My CS classes where pretty gender-balanced, more so than my high school, and I can assure you that's not the problem.