Well I've been able to grow a full (albeit moderately thick) beard since I was 17 (I'm 20 by the way), but being on propecia for almost 2 months has caused it to look a bit thinner than usual. However, I can't make a real judgement yet as it may have more to do with the changing of seasons, I'll have to check once I pass 6 months to a year.
But your not wrong in thinking that finasteride might cause thinning of facial hair as much of our facial hair is DHT dependent and by reducing the amount found in the blood could lead to thinning. The same could be said about body and pubic hair.