Get this from some outdated text? Since from what I have seen as evidence, those marrying young (the females) are usually more prone to all sorts of problems when such is less likely if marrying when older. Of course it might be different in the Muslim world - where rights are not such an issue.The age of marriage is puberty I believe. And we should get married as soon as possible (not wait after we "grow up").
And growing-up, or our development, is apparently not over until our early 20s, so even this isn't a good reason.