Premature Graying of Scaip hair

I am a 21 year old male.

I have a lot of white hair on my scalp. It started when i was around 14 years old. Now 25%, or maybe 50%, or maybe even more of scalp hair is white. Its very embarrassing.

Is there any cure for it ? Is it true that once a hair becomes white, it cannot turn black again ?

Thanks.