Hey,
To be honest it depends. Size does not really play a factor in the exact age of alterna. Sizes can vary due to locale, feeding, whether snakes are cooled or not, etc. For example, I have a 09 non locality male that was power fed and not cooled, he is approaching 27 inches. I have a 09 East Sanderson male that I purchased that I am going to say was cooled, and he is roughly 14 inches. I also have a 09 highway 277 female that was not cooled and she is approaching 18 inches. I purchased a pair of 08 East Sanderson females that have been raised gradually and they are all around the same size. It depends on your tendencies, frequency they are fed, if they are cooled down or not and locality. Good luck