I suppose that the age similarity might mainly be a peer group effect. People in the early twenties tend to hang out a lot with people of around their same age, e.g. due to college/ university classes or parties. As long as dating happens predominantly within the peer group, age differences usually aren't very big. But once online dating is used, larger age differences occur quite naturally.
What I am citing comes from surveys of men and women, not just observations from anecdotes. Women virtually always want men just a bit older, throughout their lives, or at least say they prefer this. I think you can find some of that data in link below.
