A lot of women, when they're young, feel they have very good friends, and find later on that friendship is complicated. It's easy to be friends when everyone's 18.

This quote is saying that when women are young, they typically have strong relationships with their friends and as they get older, they realize that many of these relationships become more complicated. When everyone is young, it is easy to be friends which is why the relationships often feel so genuine and strong at first. However, as life progresses, this tends to change and people realize that friendships can be complex.