though you're forgetting one thing still. When girls enter that stage where their bodies start to change and their feelings start to change with them, it is seldom a boy their own age who acknowledges that. Boys are lagging behind in that regard.
It is the older ones who recognize that a girl is entering her puberty, dressing sexy clothes and generally exudes sensual sexiness. Therefore it is the older ones that will reciprocate any advances made by the girls in question. And that includes adult males that sometimes have no clue as to what they're doing but sometimes know exactly what goes on.