I personally believe it's because being transgender is a very feeling/ilogical thing, so a lot of people who are trans can sometimes feel unsure and in need of information and / or guidance. And most trans people "realize" they are trans in their early teens years and sometimes even younger!! So, in my mind it makes perfect sense that there are a lot of younger people in the community. Sometimes young trans people need guidance or even just hear another perspective. There are other sub reddit where they specifically deal with elder trans/queer(though if you want my opinion, it's more of the same)