Are they both natural?
【We're talking about puberty】

My facial hair came IN fairly quickly and suddenly after 15.
My facial hair came ON fairly quickly and suddenly after 15.

Question

Are they both natural?
【We're talking about puberty】

My facial hair came IN fairly quickly and suddenly after 15.
My facial hair came ON fairly quickly and suddenly after 15.

Answer

"In" is the proper preposition to use in this sentence.

Answer

I would use 'came through'. We use this verb for teeth too. You could also say 'developed' if you want to sound a bit more formal.