I'm a teenager & i'm starting to have grey hairs already. i mean they're just popping up everywhere. is that healthy? what is it caused by?

Depends on pattern: Gray hair can be an aberrant thing, meaning it just happens. A few popping up here and there is fine. However, if it's coming up in a specific grouping or pattern, that's a problem. If you're turning wholly gray, you may need to consult with your doc or a derm. A lot may also depend on genetics. If your dad went gray early you may too.

Graying of hair is: related to genetic predisposition. Age of initial graying is largely a matter of genetics. However stressors: nutritional deficiencies, chemotherapy, traumatic head injuries, low vitamin B levels, zinc deficiency, severe illness, surgery & marked emotional stress may impact earlier depigmentation in people who are already predisposed to gray hair. Aging is a significant factor. In general –