If I can't grow a bear at 21, will I ever be able to grow a beard?

Hair growth is herditry. You can shave daily the portion you want to get a beard for few months. To some extent possibility of getting beard can be increased.

I would be more concerned if you actually grew the bear, as bears are quite large.

Many guys can't grow a full beard. Beard growth is a genetic trait, not a puberty trait. Only about half the males on the planet can grow a beard. So if you have your dad or one of your granddads who can't grow a beard, you probably inherited that trait. Might still get some more facial hair growth over the next couple of years but more likely you won't at this point. Most guys who can't grow a full beard or just have patchy growth usually stay clean shaven since it does look funky otherwise. Or they may be able to grow the mustache and just have that.

What does a bear have to do with a beard?