It's women and men alike.
For women, the appeal is someone who is cute, innocent, and under her control. A boy is not likely to step on your heart the way an older man would.
For men, the appeal is wish fulfillment, thinking back to when they were kids and had crushes on hot women and teenagers. There's also the aspect of M-Sub where men are tired of putting all this effort into courtship, and thus idealize a scenario in which the balls are in the woman's court.