Hello. I know a kid (13–14 years old) who has developed quite a lot of grey hair, and it’s very noticeable.

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I know a kid (13–14 years old) who has developed quite a lot of grey hair, and it’s very noticeable. Does anyone know what could cause this? I’ve read online that it can be harmless, but they also mentioned more serious conditions, which is a bit worrying. I’m planning to advise him to see a doctor, but I’d like to understand the possible reasons first. Thanks