I took finasteride for 2 and a half years straight. I was 18 when I began and 20 when I dropped. My facial hair was progressively filling in before the pills usage. However, while on the drug and currently (5 months after quit) my beard is still patchy in some areas.
Genetics is not the reason for this, since every man on my family (both grandpas) have (or had) "alpha" beards. Looking at pictures from my father's graduation, he had quite a furry face.
I'm not sure if my hormones are all messed up to a irreversible situation. If that's so, then I should find ways to increase testosterone (including its free form), keep DHT at healthy levels and (if that's possible) get a higher distribution of androgen receptors (found along the face).