How to prevent hair loss. i'm 21?

Depends: If there is a causative factor for the hair loss, treatment of the condition will usually take care of it (stress, iron deficiency, thyroid). Otherwise you hair pattern is mostly hereditary even though Rogaine and Proscar may be helpful in regrowing hair in some people.

Hair Loss Prevention: Because there are numerous medical and genetic factors involved, preventing hair loss at ANY age requires a medical consultation with a ABHRS board-certified, full-time, experienced hair restoration physician specialist. Today, this is done long-distance via email, phone or skype. Good news, effective treatments are available to help keep your hair growing strong.