I was 30 before my stash grew in properly, and by that time I was already starting to get grey in my beard. I have even been mistaken for Frank Millar, although I dont see it.
My best friend had a "Charlie Manson" beard by the time we were in grade 8, and another friend could only grow a "Don Quote" by his 30th.
Genetics seem to play a role, and this might sound racist, but some races(like Natives) tend to have far less body hair then other races(like Pollocks), I always blamed my lack of a stash on my native blood, cheek bones to die for, but spent most of my 20's looking like a teen.