Intuitively, being more mistake-prone would seem to be the explanation for having older CB's in general, but it's just as much a result of the position being less reliant on speed and fitness in general, so CB's are more effective longer, and thus there just aren't as many spots for young CB's.
Games-played is more important than age in terms of decision making, so the problem is really that since CB's can play longer, it takes young CB's longer to accumulate the necessary experience. But they have to accumulate it somehow, so to get them to the higher level, they're going to make the mistakes regardless, they'll just be spread out over a longer period, because they don't play as much when they're young. If that's true, teams should just bite the bullet and play young CB's as much as possible, live with the mistakes and have a finished product for several more years than they would otherwise.