How many milligrams of biotin should a thirteen year old girl should take?

Currently, there isn't enough scientific evidence to recommend a specific dose of Biotin. But for all practical purposes the following doses (in micrograms per day) can be used to prevent deficiency based on adequate intake:

• 0-12 months - 7 mcg
• 1-3 years - 8 mcg
• 4-8 years - 12 mcg
• 9-13 years - 20 mcg
• 14-18 years - 25 mcg
• Adults >18 year - 30 mcg
• Pregnant women - 30 mcg
• Breast feeding women - 35 mcg

Considering the age of the girl in question i.e. 13 years, a dosage of 20 to 25 mcg daily is adequate to provide for her daily needs.

However, if it is for therapeutic use, it is better to consult a physician who will recommend the dosage based on severity of deficiency.