Interesting thought. What about this: Younger guys with "equal" density to older guys may have the exact same amount of hairs growing and in the exact same places, BUT, a good percentage of the hairs are still thinner. What I mean is that not every hair is truly "terminal" and growing thick, but it grows thinner and perhaps even has a shorter length, or at least grows slower than the terminal ones. So if we take that in account, the older guy who presumably has all his hairs grow at similar thickness and even speed, will have a thicker-looking beard eventually, while the younger guy even if he grows for the same amount of time, will look less thick because of the many thin hairs and also slower growing hairs. Just a thought.