For ginseng, age and size do not go hand-in-hand. The root neck will get 1 new scar every year (unless it is dormant). But a root will grow either fast or slow depending on it's conditions. The soil type, the shade, the weather, the other plants around it all have a lot more to do with size than the age.
Some of the biggest roots I've found were 3-prongs. And there have been a few 4-prongs that were so small that I left them go, even though they were 10+ years old. Ginseng is a very crazy plant.
Sunlight, sunlight, sunlight. Take a 40 year bunch of sang in an old growth forest may only big as your pinky finger. Then take a 2-3 year old two prong, a tree falls or timber is cut and bam in 3-4 years that root will look like a poke root. I always walk straight to the light spots when I am sang hunting.
I think that for the most part, sunlight and soil conditions (specifically calcium levels) make the difference.
Last year on my birthday I hunted down a long hollow with a hillside that faced due north, deep shade, heavy timber, average woods soil (mostly clay).
The 4 small roots with long necks (20-25 years old) to the right of the 20.00 below all came from that north facing hillside in heavy shade with average clay soil. They were 8-10\" tall 3 prongs and the leaves were sort of thin and pointy. They had small berry stems with like 4-5 berries.
When I got to the mouth of that hollow and turned right, there was a limestone bluff there (200-300' tall) and around the bottom of that bluff there was a place where the grade was around 12/12 pitch, and had some flats on it. That bluff faced due east and the seng there was getting good morning sun, but was shaded well after that. The soil there was rock chip filled (probably very high in calcium). I actually collected a soil sample that I am going to have tested soon.
Anyway on that bluff I found some really nice big 4 prongs and 3 prongs. They looked quite different than the other seng - in that they were big and stout and the leaves were broad and rounded and they had taller berry stems and big wads of berries 20-30 berries.
The roots I found on that bluff are shown below and to the left of that 20.00 below. They all had like 5-8 flats on the curl.
I think in this case, the location, sun, soil made the difference.