Starting at about the age of 11 and as late as 16 years old, humans reach puberty. Muscle mass and body size change based on genetics and hormones. At the end of puberty males have more muscle mass, on average, than females with the associated increase in strength. But at the age of 12 a female would have the same muscle strength as her male peer, easily out preforming him in physical feats of strength and skill.
Seger, J., & Thorstensson, A. (2000). Muscle strength and electromyogram in boys and girls followed through puberty. Retrieved June 18, 2015, from http://www.ncbi.nlm.nih.gov/pubmed/10552267