When I was 16/17, many of my friendship group were dating 22-25 year olds. Some of us married them and are still married. This was mid-1990s. No-one batted an eyelid. The boys in our sixth form were dating 14/15 year olds. We wouldn’t have looked twice at boys the same age. I guess times change.
Times change, pendulum swings, maybe even too far. My guess that those former teenagers who dated over 20's - many of them had a not so good experience and don't wish it on their own kids. And happy ones are ignored.