Corns have had their most of growing when they are about 3 years old. After four years you know pretty much how large they are gonna be. The older they get, the slower they grow, but they do keep growing a tiny bit. Not sure if a snake that was fed too little, would still make up for it, even after reaching the age of 3. I do have some smaller snakes though that grew quite a bit after I bought them at about 2-2.5 years old. I think they were fed at the more modest side of the scale, and at my place they were fed more often/larger preys.They seem to have a growth burst after changing to a larger prey.
Well, males can usually fertilize eggs when they are 1.5 to 2 years old, females are able to reproduce at that age also. However, especially females are being bred later to give them time to grow and become adult length- and weight wise. Many breeder use the rule of thumb that a female should be 3 year old, 3 feet long and weigh 300 grams to be bred, though others just look at their body shape/length and muscle tone to determine if she can be bred. I guess both types of breeders would consider the female to be adult when ready for breeding.
I consider both females and males adult when they have been bred (succesfully) OR are 3 years old, which ever comes first.