How can i tell the age?

About two months ago i got my very snake, Java. When i got him he was about 12 inches, and now he is about 18-20 ish, he wont sit still long enough so he's making me estimate. And i got have the scale thing to weigh him. Is there any other way i can tell? ( i got him at Petsmart and they were no help)

Honestly there is no way short of knowing when he hatched. You can guess and probably be within a few months of being right, but BP's can grow at vastly different rates.

Chop him in half and count the rings! KIDDING!!!

He is less than 1 year, but how old exactly is impossible to guess.

Like others said, there is no way to know how big the snake is. If a snake is underfed or doesn't eat well it can easily look 1-2 years younger than it actually is. They all grow at different rates depending on how much they are fed/eat and their own growth, so length/weight can't tell you much about their age unfortunately.