Young girls are naturally more desirable than older women. It's simple biology. Men who disagree do so simply to comply with societal norms. It's sad, really.
To explain, let me list some of the traits that men typically look for in the perfect girl.
The perfect girl>is small>is cute>is pure>is impressionable>is soft>is naive>is innocent but has a natural curiosity for lewd things>smells good (faint milk/crayon/fruit smell)>will always look up to you>will listen to you>will trust you>is interested in your hobbies>doesn't care only about money>likes to watch you play video games>will let you teach her all kinds of things>likes to cuddle with you>has perfect skin without blemishes
As you can see, all of those exclusively apply to younger girls. Also, they tend to have a slightly higher body temperature, which feels great when cuddling.