I have to disagree that all young guys are immature and not intellectually equipped to stimulate a woman's mind, emotional needs etc. I think there's plenty of highly intelligent and mature young men all over the place, and I know there's plenty of fellas my age (I’m in my 30's) with the mentality of 15 year old children... So I think there's more to it than that.
A woman dating an older man = slightly taboo = more exciting = more of a turn on (for some), ie. If the older man is 40 and the girl 20, people will talk and it will attract attention!
Secondly, older men generally have more money, their own property, their own car, all this = much more freedom and ability to do those "dreamy things", spontaneous holidays, gifts, etc, which is a massive plus! Additionally older guys have established themselves (perhaps senior position at work, that demands respect) - powerful men = more attractive! An older man is not concerned what his parents think or feels he has to answer to them.
They also (in theory) know more about sex and how to please a woman, know more about a woman's emotional needs and (if required) can provide that parental type guidance that a woman may also seek from her Father, taking a dominant / confident role in decision making pertaining both to the relationship and “life guidance” for their partner. So in summary the extra security, freedom and power makes the older man an attractive alternative to the young less established "stud".
If the older man is in great shape with a high sex drive then it's probably a win win situation for the young girl!
As a side note, funny thing is alot of older divorced women in their 40's date younger men - it's a reversal of the above... and the young guys love it for very similar reasons. The older woman offers more security, freedom and is more established. So it works both ways. I think it just pisses off young men more than it does young women – lol!