Two factors. First of all men's and women's gymnastics have some different events and different judging criteria for the events they have in common. On the whole the men's events and scoring criteria require more core and upper body strength to pull off successfully.
Secondly while the strength to weight ratio of women might peak at 15-18, in men it keeps increasing into their mid-late 20s. In general, the strength difference between a 15 and 25 year old man is much much greater than that between a 15 and 25 year old woman. Basically a 20-25 year old man can pull off feats of strength that a 15-16 year old simply can't. The same isn't really true for women (again in general).
