natural adult teeth discolouration

My brother had normal baby teeth when he was younger but as his adult teeth came through they were discoloured a great deal from the beginning, they have a dark brown stain on the first few teeth at the front of his mouth.  He is now 19 years old and his teeth are still stained.

I would like to know if there is a medical/dental reason so many of his teeth were discoloured naturally?

[pacecar]

Did your brother take many antibiotics as a child?   This can cause discoloration of the adult teeth (specifically tetracyline antibiotics).  Also, is it possible that he is now smoking and causing further discoloration?  Best of luck.