No way to tell really. They're not like trees where you cut them and count the rings. A question that would need to be answered is how long you had the snake and how much has it grown with you. Burmese pythons are known for reaching 10' in one year. However if you have not had the snake long and it's the same length now as when you first got it it may not have been fed very well when it was younger and could be 20+ years.