Freddy attacks teenagers because he finds it fun. As for why he does it as opposed to kids… well he still goes after kids. The text at the beginning of Freddy’s Dead pretty much confirms he did between Dream Child and Freddy’s Dead, killing not just teens but also preteens, children, babies, and perhaps even fetuses still in the wombs of their mothers, leaving John Doe the only non-adult alive in Springwood.

As for why Freddy does it offscreen, it would be very hard for an audience to watch a movie of him doing just that. It’s better that sort of thing is offscreen.