How much biotin is a 13 year old allowed to take?

You should not be taking supplements without your parent's or doctor's supervision.  Biotin does not grow hair.  The Internet may be a great source of information but it can also be a source of incomplete information leading to confusion and misunderstanding.  

At 13 you are very young to be so concerned about your hair that you have taken to the Internet to ask the advice of strangers.The first step, if you have not already done so, would be to discuss your concerns with your parents.If they agree with you then they an take you to see a specialist who can advise you on treatment.Deciding to take biotin randomly is not the correct approach.

Biotin will not help most 13 year olds in any way with hair growth because it is rare beyond rare for a children and adolescents in North America to be biotin deficient unless they have specific genetic conditions that affect biotin metabolism.If you have hair loss, be sure to see a dermatoligist who specializes in hair loss to get a diagnosis to understand why you are losing hair.Then you can discuss how best to treat.

Taking Biotin and anything in large doses is not a good idea. Read the bottle and see what is safe and what is not. If you are losing hair at 13, then you need to see a doctor as this is very important to diagnose and treat properly, not by using your own home grown treatments.