But people don’t experience attraction before puberty, right? How do they know what groups of people they’ll grow up to be attracted to?

Well, we all have crushes when we were kids, don’t you agree?

Edit: sorry if I sound like I am trying to put words into your mouth. What I mean is that some people already figure out what their sexuality is and whom they feel attracted to at early age. To whom you feel crush to at early age is an indicator.