Mortis...
Right here close to my home I see the same thing as you... almost all 3 prongers. Very rare to find a 4 prong (only found one in many a year hunting here close to home).
Found a many 3 prong in the 20-60 year old range that were still smallish 3 prongs (8-10\" tall).
But now on down the hollow a mile or so, near the creek, and a limestone bluff I wondered one day hunting and found a nice pile of big stout 4 prongs with beautiful bulb roots. The roots were 2-3x as large as what I usually find closer to home and these plants were not all that old either (most looked to be 5-8 years old).
They were on a east facing bluff, rock chip filled soil, getting a lot of morning sun.
It's the location, soil type / soil fertility, sun exposure, moisture, etc... that makes em get big and stout - not necessarily the age.
Sounds like where you normally hunt you may have conditions close to what I have near home here.
I would recommend that you search out areas that you might get to hunt that have rock bluffs facing north to east. I always find really good ginseng, and lots of 4 prongs in areas like that.
Good Luck
TNhunter