Question

why boys starts developing facial hair

Answer 1

because of their puberty
it comes under the secondary sexual characteristics thnx

Answer 2

hello....
here is ur answer....

In teenage boys, facial hair grows thick and darkens because of the effect of testosterone. ... Whenboys enter puberty, their testicles begin producing more testosterone. It's responsible for many of the changes your body goes through. Getting a beard is one of the changes brought about by this hormone.
hope it helps u...