I am mid- twenties and keep growing grey hair is that normal and how can I prevent it.?

It can be normal. 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.
Nutrient deficiency. Nutrient deficiencies like biotin and other b vits can lead to premature aging/graying as can built up environmental toxins and heavy metals. Go to a4m.Com and use doctor locator to find a functional medicine doctor near you to help evaluate with specialized testing for nutritional deficiencies like metametrix organix profile and help you correct them.