21/male hair thinning a little and facial hair not growing as much as i'd like would rogaine (minoxidil) help with these? any bad side effects? any tips? etc.

Start treatment now: for what may be early male baldness. You will have much better results by using minoxidil now. Hair loss at an early age can be devastating to your self-esteem, and the longer you wait the harder it is to gain the lost hair back. Some formulations of Rogaine can cause scalp itching, so try foam if that's a problem. It is not approved for beard growth, but some men on YouTube have tried it.