Young girls are able to beat boys in wrestling because there are no noticeable strength differences between the two genders. Prior to puberty, girls and boys are competing on an even footing when skill is the deciding factor.
After puberty sets in, the strength differences between the two genders becomes more apparent. Wrestling is a weight class sport, but an athletic male is likely to carry more muscle mass than an athletic female of the same weight. It is rare to see teenage female wrestlers performing well against their male peers unless the female wrestlers have significantly more wrestling experience.