I agree with blessings comment on if someones really immature they shouldn't be getting married to anyone.
However, there are quite the few exceptions where the g irl is immature but ready for marriage. i'v seen a couple of couples where the wife is a lot younger than the husband, she's immature but responsible enough to make a marriage work, and the husband though older and maturer not a total old goat, and young enough to chill out. they balance each other.
Personally? i don't think age matters, unless a man is like 80 and the woman 20 or 30, or unless they are totally incompatible. otherwise, age doesn't matter.