Well, most but not all, take Stephen Gogolev for example: we have videos of him jumping quads in 2016, when he was like, 12 or 13.
Also, girls mature earlier than boys. For example, take the puberty age - for girls, its between 10 and 14, and for boys, between 12 and 16. A difference of 2 (!!!) years. Or compare the growth curves for boys and girls:
https://tall.life/growth-chart-child-boys-girls/ again, we have huge difference: boys stop growing around 18-20, while girls stop growing around 15-17.
"but most start learning quads when their bodies/bones already mostly matured, at 15" you say. First, the boy's bones and body are NOT anywhere neat matured around 15. And second, if this growth stage is acceptable for boys to train a quad, the same maturity stage of bones and body is achieved by girls way earlier. A couple of years earlier.