EvoPsych Reason: Younger women (although if you get too young, this reverses, obviously) are more likely to bear healthy children. We are the children of the men who had the genetic predisposition to pursue mates that produced healthy young.
Status Reason: Most women prefer men around their own age. If a man can get a woman significantly younger than him, that indicates that he must have high value in other areas of his life. See: white-haired millionaires with a girl younger than their daughter on their arms.
Dominance Reason: Because younger women are perceived as more pure and less experienced. This means that the man believes he will easily take the power in the relationship and there is something exciting about defiling something innocent. But that makes us sound like monsters, so lets ignore that one.
Attraction Reason: Firmer and perkier.
Provider Reason: Many men enjoy being in the role of a provider. They like knowing that they can bring resources to those that they care about. Such men might purposely seek out women who are still putting their lives together so that they can live in that provider role.
I could probably keep going, but I think five reasons is a good start.
Because men do not want women who are more mature than they
They like them as young as the law will allow. That's just human nature (As a general rule).
i think it's statistically a sociobiological thing - in practice sometimes it can be a quite evolved, spiritual/mentor relationship - quite often though it's just men refusing to grow up
type of complex by the women around their age