I noticed when I was in my mid 20s that some people got grey earlier than others and some were really prematurely grey.
I started asking these females and males about whether they had dyed their hair when they were younger? It was an observation that a lot of people had dyed their hair and gotten grey prematurely and the answer was always yes.
Well I found this article – interestingly it validated my observations.
See article on Hydrogen Peroxide and Gray Hair
For more about healthy hair