According to "common wisdom" the average man has, as a proportion of his body weight, 50% more muscle mass than the average woman. This is in addition to how the average man is several inches taller than the average woman and thus has a larger body mass to take that proportion out of.
Notice the above is phrased in terms of men and women as opposed to boys and girls. I am interested in how strength compares at a younger age. If we took the average ten-year-old boy and the average ten-year-old girl $-$ before the first growth spurt of puberty $-$ and compared their proportional muscle masses, how would they compare? Have any studies been carried out?
Looking for information online about this nets me a hundred accounts from men who remember how, as children, they lost every fight with their older sister. The problem here is that in any contest between siblings (twins excluded), the older sibling most likely has a height and mass advantage. So it's hard to say much about proportional muscle masses.
My guess is that if we took twins Bill and Roberta then they would have similar muscle-mass proportions up until about age 10. From then Roberta would have a growth spurt and increase proportional (as well as absolute) muscle mass due to extra testosterone (in addition to extra estrogen). A few years later Bill would have a growth spurt and increase proportional muscle mass to exceed Roberta's. But she would still be stronger than him, until he caught in terms of height.