A more important question: why are there so many guys who remain immature into their late ’20s? If a 26 year old man (not a *boy*, a *man*) is on the same maturity level as a 19-20 year old girl then he has got some very serious problems and shouldn’t be getting married at all until he gets his head on straight, peferably with some professional help.
Another angle: women significantly outlive men of the same age. A man who marries a woman 15 years younger than he is will leave her a widow for at least 20 years. And, if they have children, most likely she will be left with the youngest one or two to marry off. That isn’t romantic at all, that’s just cruel selfishness, both to the woman and to the man’s own children.