I think a major cause is that today’s 23 y/o boys are underqualified when compared to thier 21 y/o girl counterpart. So rather than ramp up thier own standards (get a better education, job prospect, ect) they prefer to match wits with someone who is on thier own level (a 19 y/o girl who is impressed with bein ha’zmanim trips and things like that).
Hopefully, my boys will be better prepared, and find that they only feel comfortable with someone thier own (close to) their age