Actually having lived in many places in Japan, I would say this is more a Tokyo thing than a national thing. My friends from Tokyo are in their 20s and the majority are virgins who always lament the opposite sex never liking them despite not trying at all themselves, whilst those from Okayama, Fukuoka, Okinawa, Hiroshima, Kagoshima, Aomori, Hokkaido etc. have been fucking since high school (I personally know three girls who had kids either in late middle school or during high school).
Kids are all over the place where I'm living at the moment (Kitakyushu), and I see plenty in Okayama when I visit, but when I spend any time in Tokyo you barely see any anywhere. Birthrates say so too, whilst the country as a whole (apart from Okinawa) is below replacement rate, Tokyo is ridiculous, having 1.06 kids/woman when the next lowest is like 1.3.
Others are right re sex education though, it's not a thing here really at all. When I was in high school they had a whole school assembly where one 60 year old woman gave a talk on how babies are made, and she had a purple felt penis and a green felt vagina to point out where things were. Didn't actually say anything about sex beyond that cum makes babies, before making one of the 'cool' baseball players go up and go elbow deep in the felt vagina to pull out a terrifying felt baby. That was the end of it. My fiance didn't even know oral was a thing until I tried to go down on her (when she was 19, in the west you know of that shit when you're in middle school, if not younger)







