Why do I have grey facial hair growing at 16 ?

It's possible that you have a nutritional deficiency. Nutrition is still a young science, not thoroughly learned, and the effects of combinations of vitamins are not studied at all. But it is known that B vitamin deficiency can cause graying hair. Fortunately it's cheap and easy to test: take pills and see what happens. You will have to study some books to figure out what pills to take and how to eat right in general. Doctors can not advise you because they have no such training.

The most popular introduction to the field:
www.amazon.com/Adelle-Davis/e/B001J3RULO