Snakes, and most reptiles in general, are difficult to determine age. They grow at different rates from each other and it depends on how much they eat, genetics and a host of other variables. However, when you've gotten one young, as you did, it's easier to take an educated guess. The 3' 3 years thing is more in regards to when it's appropriate to breed a female, if she's at least 3' 3 years and 300 grams, she is okay to breed, not necessarily that a 3 year old will be that size.
Most corns hatch around April to July and later clutches usually hatch around Sept or Oct. Based on that, I've determined that my pet store snake is either a late hatch '07 or an early hatch '08, as I got it in Sept '08 and it was about 15" and looked at least several months old. But anything closer than that is pretty much impossible with snakes if you don't have an actual date.