All the stars have to align to produce a B&C buck. I've spent a lifetime hunting trophy antelope in Wyoming. Depending upon the year, in the best quality pronghorn units Wyo has to offer there may be 1 buck in 500 that are B&C. There may not be 500 bucks in some Colo units!
Age is just a small portion of the qualities that produce booner bucks. In fact, Grimmet has conducted summaries of B&C bucks and found that pronghorn can reach B&C minimums at 3 1/2 years of age. Elk and deer may take 7 to 12 years to make B&C! Genetics, moisture, water availability, mild winters, age, nutrition, predators, etc are all factors that produce whoppers. Colo may have older age bucks in the limited units but there is definitely something missing....possibly genetics?
Just because a unit has limited tags doesn't necessarily mean it has older age class bucks. If you use NW Colo and North Park as examples...both of these areas produced a small number of B&C bucks 10 to 30 years ago. With drought plus winterkill in those herds every couple years the bucks never age.....even if there are only a small number of tags issued! There are only a fraction of antelope now than there were 15 years ago. Back to back to back years with poor habitat, severe winters, lack of water, predators, etc have all lead to the decline in antelope numbers, likely younger bucks, and fewer B&C bucks in that corner of Colo.