What’s wrong with that.
What’s potentially wrong with that is the boys leaving Eretz Yisroel before they’re ready. The whole idea of having boys marry earlier, even those who are mature enough to handle marriage, is not simple at all. When someone gets married, he takes on financial responsibilities that he did not previously have, and in the vast majority, getting married younger means leaving full-time learning younger.
Whether or not the age/gender discrepancy warrants this sacrifice is only a decision that the gedolei hador could make, and each bochur, with his rebbe, could make.
Encouraging considering slightly older girls to marry does not have this drawback.