White hairs in my beard at 15 years of age

Hello

So I am 15 and pretty mature. I look about 19 years old.

So the problem I am facing is, that i have couple of hairs in my beard is white. I literally have white hairs and I am 15!!! Majority of my beard is black otherwise.

I smoke tobacco and ocasionally smoke weed and drink, if that's related.

What's the cause and how to prevent it?

Thanks