For me, my beard was actually much thicker in my 40's that my 20's. But since you started this thread with the comparison between those with relatively the same density of growth, I would say it is the younger man's desire for instant gratification. Older guys have learned that any achievement takes time. A teenager I know has told me several times he wants a beard like mine. The fact is, he has a beard the is much thicker than mine and would be much better than mine in a short time. He goes maybe a week or two and has a very good start, then trims it down to some style such as a thin chin curtain. A week later he shaves it all off. He has never let it go long enough to realize his own potential.

My Beard Journey:  http://jefffsbeardboard.yuku.com/topic/25137/Finally-I-have-a-Beard
Beard and Bald Thread:  http://jefffsbeardboard.yuku.com/topic/27831/Beard-bald-head-shaved-head-or-buzzcut
Patchy Beard Success Stories:  http://jefffsbeardboard.yuku.com/topic/33742/Patchy-Beard-success-stories-Before-and-After-photos#.ViujbH7nuM8
Taming the Wild Mustache:  http://jefffsbeardboard.yuku.com/topic/30822/Taming-the-Wild-Stache#.Vj_Mh7erTIU