What appeals to someone sexually is very very hard to figure out. We're all wired differently. Some guys dig tattoos, some hate 'em. Some like tits the size of basketballs, other guys prefer mosquito bites.
Here's my guess and observation as an amateur psychologist.
Ever notice how most people tend to always love the type of music and songs that they were into when they were in their late teens early twenties? Those songs trigger memories of what is often the best time of your life. You're young, finding your way in the world for the first time, exploring things, and generally care free. You live kinda like Jace does today.
My theory is girls of that age push those same buttons in us psychologically. Takes us back to those times. Let's us have the fantasy of being that young again.
Or half the world are closet pedophiles...