HayyimOvadya,
Most 18-21 tear old boys are discouraged from going out because they’re not ready, so I don’t know if that theory can be tested. I know of cases where the husband is a year or two older, and I hear it’s becoming more frequent.
Where there is reluctance, as I said, I think it’s more from the men’s side, both because they consider it more of an issue than the women do, and because they are in a better position to be more specific in whom they seek as a spouse.