Should teenagers be proactive about their hair i.e. can they take propecia as a safeguard against losing their hair in the future especially if both parents have experienced hair thinning? At what age can young men take finasteride?
The best way to be proactive is to get serial mapping of the scalp for miniaturization. When early hair loss occurs, the use of finasteride may be a good idea. You will need to get medical attention for this process.
Propecia studies that lead to the FDA approval were conducted on men age 18 and up, but if a doctor should prescribe to a 16 year old, for example, that is up to him/her ultimately. I would never recommend just popping a Propecia pill each day or applying minoxidil if you have no hair loss.