It depends on genetics; some can at a young age (I knew a guy who had a full 'stache & beard at the age of 12), some can't till their mid-20s, some never can. I'm of that last persuasion, it just doesn't grow in thick, coarse, or dark enough. While I can grow it long enough to look like a full beard, it doesn't have that 'lumberjack' look, more of a 'hippie look'... which I don't want

I'm otherwise quite apeishly hairy, so it's kind of weird that it doesn't much grow on my face. Asians are especially likely to be like this, but I'm mostly Italian, so it's not limited to any genetic type.