Don't know. I let my beard grow at 21 at it filled up all over. But since I never tried to grow it out before that, I don't know exactly by what age before 21 I would have been able to have a hairy face if I tried. My gf at the time wanted me to have a beard, so I grew it and kept it while we went out. Now, I pretty much grow it or shave it depending on the preference of the person I date (if they express a preference on the matter).
Some guys won't be able to grow facial hair. Others yes. For me, it takes about 8-10 days before I look like I've been stranded on an island for a month. Good luck with yours, if that's what you want. But I really don't think it matters that much.