Women who have kids and are over 35 are not allowed to be attractive to people or something? The fuck.
Pretty is pretty. I don't care if you're 18 or 50. If you're pretty you're pretty. And if you're pretty I WILL want to fuck you. Guaranteed.
MILF is just a convenient word people use to describe older women who are bangin'. There's nothing scientific, philosophical, or psychological about this. Jesus Christ.
Also, a lot of guys just aren't into "little girls". And what I mean by that is not under-age girls, but young women who still have the high-school mentality or are just inexperienced sexually. That shit is a huge turn-off to a lot of men believe it or not. A lot of men like a woman who knows what she likes, isn't afraid to ask for it, and seem to be on their level sexually.
edit - Also, a lot of MILFS (in porn) tend to have big phat titties and nice asses and coke bottle figures. I don't know how much you guys know about men (I'm an expert) but phat titties and asses is pretty awesome to the majority of them.