It's genetics. I had some gray hairs in my teens, too. I just got some Revlon Colorsilk for $3 per box and dyed my hair since then. Covers all the grays and problem solved!

I also had gray hairs in my twenties. I tried to take supplement of B12 and B6 that may help restore hair to its original color due to deficiency. It didn't help, guess it's because genetic greying cant be reversed..

I think both low iron and chronic stress can make your hair go gray. I would try getting them both treated with an oral supplement of iron, some lifestyle changes, and maybe some form of psychological support, as it would help you in a lot of ways beyond your gray hair. Be well!

Not an expert, but when endogenous levels of antioxidants are low (glutathione and superoxide dismutase (SOD)) greying can happen faster. I’d also check your magnesium. You need magnesium for glutathione production. There is something called the glutathione / magnesium diad. If one is low the other may also be low, and to raise one you may have to raise the other simultaneously. Which you can do with NAC + magnesium or glutathione + magnesium. NAC n-acetyl-cysteine is usually cheaper Just some thoughts from a non-expert.