Average penis size for 13 years old?

It varies with the time of puberty. So if you start puberty at 9 then at 13 you may reach a peak penile length and the average is about 5.2'' for American. However, if your puberty is much later, like 12 years old, then the penile length will be probably less than 4" and it will grow til you reach 16-17 years old. So because there is no “normal” age of puberty, then there's no “normal” penis size.

It depends on whether the person has achieved puberty or not. There is no standard measurement for that age