Why do gray hair appear?

I'm worried because the number of gray hairs has increased noticeably lately.

Before, I'd only see one or two strands, but at some point, they started increasing exponentially, which was frustrating. Why do they suddenly appear as I get older? Could stress be the cause?

I was wondering if there were any supplements or foods that might help.

I've heard that brewer's yeast is good for you. Will it be effective?