When a boy is still young he perceives girls to be gross and weird, therefore he would only associate with other boys. Girls are the same way at a young age, they think boys are way to different and are just plain weird, therefore they only associate with other girls, but as a young boy or girl starts to age and develop they start to notice things about each other. Both boys and girls hit puberty and they both start to develop and consequently they start to observe things about one another. These young men and women went from perceiving each other as weird and gross creatures to being attracted to one another. Gender is not the only thing that you perceive differently as you age, you start to perceive social norms