Corns are usually about 12 inches long when they hatch and double that length in the first year if properly fed. That works out to an inch in length per month. On the other hand, snakes in the pet trade are often not properly fed. If you can get the snake to stretch out against the back of her cage and measure that distance, you can get an approximate minimum age. This is only good for the first year, though.

Just from the picture, I'd guess that snake is this year's hatch.