I couldn't grow a decent full beard until I was about 20, but as has been said before, some men are never able to grow full bears. Genetics play a large part.
I started shaving when I was about 16 but it was mainly just my mustache and chin. I probably couldn't grow a decent beard until I was about 25.
Like most have already pointed out, most guys aren't able to grow a full beard until their 30's or so, but then again it all depends on the person and their genes. For me, I could barely grow a full goatee at 20, the rest of my face was incredibly thinned out. Then around my mid 20's or so I started getting the salt and pepper effect, now at only 31 my mustache and chin grow out the thickest while my cheek and neck area tend take a little longer to fill out, plus the majority of the hairs are white which make it look even less full.
I think the longest I ever went without shaving was 2-3 months tops and I really could not compare my whiskers to your average red or brown haired bearded gentlemen.
One full moon, now if it only worked for my head.