Whether it is a boy or girl they are held responsible if they steal.
Puberty isn’t for common sense things. Puberty just means salaat and other obligations become Wajib.
A girl and boy have to both be praying by the time they are 7. According to the majority view it becomes Wajib for girls at 9 and boys when they go through puberty. They have to do it to the best of their ability.