Japanese people specifically have a genetic condition that makes it so that they easily get nosebleeds when their blood pressure goes high. Hence the trope of a nosebleed when a teen sees anything sexual.
I'm not sure if it's the same reason or just related, but they also have a condition that makes it so that their cheeks turn really red when they drink a decent amount (blood vessels closer to the skin in the cheeks) which is also where that trope comes from!