How to Determine Someone's Age Legally?

I visited my dentist yesterday and we talked about the age of the Chinese girl HE and her age. Dentist believes that she is under 16 years old based on her teeth. She showed 4 baby teeth when she smiles that is very very highly unusual. Baby teeth fell off before 16 years old. Even she was pumped with hormone she might still have 1-2 left. She has all of them. We are talking about the entire Chinese female team here.

I am just wondering if there is a medical method can determine people's age.