Cool. I just ordered a nice looking late 2003 yellow rat from Terry Parks.He arrives on Friday! Terry is actually looking to get rid of a proven breeder pair so maybe you might want to contact him. I had an early 2003 yellow rat that was about 26-28 inches long before he escaped on me about 6 weeks ago! As you already know rat snakes are very good escape artists and will always test the lid of the enclosure they are in looking for a way out. Not to bust your bubble but most yellow rat snakes at local pet stores are wild caught. I would definately keep him away from your other snakes for awhile. Get a fecal exam from a vet to make sure it is parisite free.
The pet stores in my area sell yellow rat snakes EVERY year and they are ALL wild caught. Sometimes they have some very high yellows but most of the time these guys are dull looking snakes. Not the high yellows you will find in a lot of CB snakes. At about 3 feet my guess is he is about 3 years old. Depending on how thick the snake is also. Most rat snakes dont put on any girth till about 2-3 years old then they thicken up pretty quick. Most yellows will get about 4 1/2 - 5 1/2 feet but some giants can get about 7 feet. Note thats VERY rare but it does happen.