Same issue here, at 28 I have bald patches of beard which refuse to fill in, but I have a full head of hair. Apparently these are related? Men who can grow beards from a young age tend to go bald on top.
Anecdotally my grandfather, currently at age 93, has a full head of hair, and yet I've never seen him with a beard. Need to ask him if he can grow one, come to think of it. I've seen him with stubble after a few days (as we're looking after him due to his poor health), but there's definite patchiness going on. They say you follow your maternal fathers side? Not sure if there's any logic to that or not.
Annoyingly my father could grow a full, thick beard at a very young age. And now he's very thin on top, so horses for courses.
I have standard stubble like guy ritchie however just growing in very slow, late developer and all that, people think I look young.