Hmm i dont think girls are more mature than guys at the same age...I tend to think guys are more logical and interested in things that matter, rather than how their hair looks or if they should wax.
Girls are more mature emotionally, sure. Probably because they live in a society where it is ok to express your emotions as a female, but socially they are very different.
Alot of males have extremely close friendships like the whole " I would help you bury a dead body in the desert" type of friendship.
Girls seem to form friendships as a security net, and although their friends might be fun to hang out with, they bitch about each other.
Girls tend to be more of a social butterfly with lots of "friends" [i.e people they know and get along with] where as guys form less friendships that, atleast for appearances sake are much stronger.
Guys tend to be much more concerned with world events, politics, literature, history, sport and whats on tv. This makes a conversation much easier to have with a male who actually has an opinion on most things.
So why do girls go for older guys?
mainly security, older guys are bigger, generally making more money, might have their own place, can drive and are infact more mature than younger guys[making the maturity gap between the guy and girl even larger]
why do older guys go for younger girls? because they are hotter, innocent, have odd views on things and are emotionally on similar levels [instead of the large discrepancy between people of the same age] girls as they get older also tend to get alot less fun.
Take this with a grain of salt, but its basically my perspective on age differences in relationships. My personal preference is varied, but it is usually someone my age or older, because conversations seem more meaningful with older girls.