@NGOtaku
When I was 12 years old, we've got in school 3 guys (same, 12 years old), who looks like Gojou. So I wouldn't be so sure, that its SO unrealistic, especially at 15. The thing is, when you accelerate so quickly at 10-12 years old, this is top where you get. When everyone around grow up after, maybe teel their 20th, people like Gojou stays the same.
For example, our guys, I mentioned, was at the age of 12 around 190-200cm tall, but they stay the same since then, not growing at all. And in the years a few guys, who was like 170, grow up to 200+, overgrown them. Biology and genetics, what can you do.