So you’re assuming (as is very popular, for better or worse) that there are more single girls then boys. What makes you think it’s not the other way around? And there are certainly many unmarried boys and girls, but do we know that it’s a larger percentage than it has been in the past?
In one publication recently, when someone wrote in saying their son’s rebbe did not encourage younger shidduchim, one advocate responded by asking for the name of said rebbe, to avoid him. There’s a huge difference between “when the rebbe supports it” and that.
Even with agreement from a rebbe, do you know of many boys who were dating at 21? From among my friends, (when I paid attention) I can only think of one girl (22) who married a 21 year old, and I hear it was family policy, not based on age gap.