There’s a major problem with encouraging boys to simply date and marry younger.
They’ll naturally want to marry girls their age (20-21) or younger, anyway, and all you’re left with is lots of young couples who are struggling, both emotionally and financially, as they aren’t mature enough to have a relationship independent of their parents and don’t have money because the boy is learning and the girl is halfway through her college degree and they now have a baby, and she can’t find a job that pays well enough to support the family AND pay the babysitter, and then what?
What we should be doing is NOT encouraging younger boys to marry; It’s encouraging boys to marry OLDER girls. That way, the girl already completed her degree, has an income, has the maturity that is brought about through life experience, and can pass that on to her husband, who should be grateful he can now learn in kollel and be supported.
Take it from me. My wife is several years older than me. And not only is our marriage amazing (6+ years, so far), our age difference has never been an important factor in ANYTHING.