|Sadly, self-injury can and does affect anyone and everyone. Regardless of age, race, sexual preference, gender, social status... it doesn't matter. Don't assume that just because your friend is a rich white girl that the scars on her arms aren't from self harm. You can never tell-until you ask.
Anyone can self-injure, even if they seem happy. Sometimes it starts as a child, other times they don't start until they are in their mid-30's. They can start at any time for any reason. More often its teenage girls that you hear about, but that doesn't mean that just that group does it.
If you suspect that a friend/family member/loved one self-harms, ask them about it. Tell them that you're worried, and that you care about them. Try to help them, but don't force them to get help.