Cut him in half and count the rings? ( I kid...)

In all seriousness, ball python growth rates will vary by the individual snake as well as the care and feeding schedule. As the previous poster said, if fed properly a ball python will grow quickly for the first two to three years, at which point the growth rate slows down to almost none. My females are generally reaching 1000 grams around a year old being fed appropriate sized prey every 7-10 days.

If the snake was underfed while young, it may not have reached the full growth potential by age 3. A perfect example are some snakes that I took in about a year ago. They were all at least three years old, but were fed very infrequently and were generally just not taken care of. I had several 3+ year old snakes that came to me under 300 grams. I have been taking care of them for the past year and they're all putting on weight nicely, up to 800+ each at this point.

As was said before, we really can't even begin to guess at an age without a lot more information. Where did you get the snake? How long had the previous owner had him? Where did THEY get the snake, and at what age?

Can you get a picture of the snake with something else in the pic for size reference?