Kids are starting puberty younger and younger. Average age now is 11. Puberty is when our sex hormones start producing themselves (testosterone or estrogen.) These hormones account for much of our interest in sex. So a 13yo will be well into their period of sexual desire, discovery, and experimentation. Did we need anyone to tell us we liked naked women pictures? That same interest extends to homosexuals as well.
For whatever reason, some like opposite-sex, some like same-sex, and some (like myself) like both. But our sexuality isn't static and unchanging and is on a spectrum. As a psychology-trained person I think nurture accounts for much of it. Are undoubtedly biological realities tht contribute to sexuality, but ultimately we're not automotons and every sexual experience is the result of choice. We be inclined to one end or the other, but how we're raised, by whom (man and women or single moms, single dads, etc.) will play a part as well. Not to the extent though any thing acts as an indicator or predictor (every gay man a generation or two ago was raised by a husband-wife combo let's not forget.)
Think what's going on with people's difficulty accepting young teens as gay is simply their not understanding sexuality, especially their own let alone anyone else's. If you don't understand yourself, you're never going to udnerstand anyone else.