Should I start Propecia? I'm 21 and I don't think I show signs of hair loss. (Photos)

Using Propecia for male pattern baldness is very effective.I would wait to start Propecia until you see significant changes. Monitor your hair and if any changes occur, bring it up to your physician to discuss your options for treatment. 

Finasteride is FDA approved for male pattern balding. It does not have any effect on normal male hairline maturation. You may want to check with a physician of you do in fact have signs of early male balding. If not, there is no indication for Propecia at this point. The best thing that can be done in such situations is frequent scalp photography like you have done here every 4-6 months.

Your biggest enemy is the progressive thinning leading to a balding pattern that is not evident at this point in time. The degree of thinning that you may be experiencing or the recession that appears on your photos would suggest that you should see a doctor who most likely will get Bulk Measurements of your hair to determine the degree of the thinning that you are presently having. If the diagnosis is early genetic hair loss (the most common cause of hair loss in young men and you are probably following your father's balding problem), then the best treatment would be the drug finasteride which is highly effective in men of your age and may not only stop the hair loss but possibly reverse it. If you are developing a more advanced balding pattern the best test to get is the HAIRCHECK instrument test which will determine the Bulk Measurements of your hair and this will show, over a one-year time frame, how effective the treatment will be. Once you know this information, then you and your doctor need to develop a MASTER PLAN which will define what you need