I've seen populations of concinnus feeding on fingerlings at a fish hatchery and they were all gigantic (3-4 feet) and fat. Steve has a point. In most natural conditions, I find they are usually about 2 feet on average. So yeah, size is no indication of age.

Skipping brumation and good feeding can make them big at a young age. I preferred to keep my last WC concinnus at a natural growth rate. They didn't get pampered and only occasionally was the winter rest skipped. They were about 10 inches when I caught them. They grew rather slowly compared to most CB snakes once they reached about 2 feet, but the male lived to be 18, the female was nearly 21 and they were close to 4 feet long. In the last couple of years, you could tell they were old, it showed. In most cases and (garter) species you can probably expect 10-15 years of age in captivity but varies. sirtalis are usually longer lived than say, ordinoides.

Young snakes grow fast but once they reach adult age, pushing for maximum size asap has a high potential of shortening their lives in my opinion.