It's unfortunate that the various natural factors governing the growth rate of a snake, such as availability of food, type of food, weather and who knows what else, make it almost impossible to accurately guess the age of a wild-caught snake. There are certain indicators.... if it's a really big specimen, it's probably quite old. If it's very 'clean' and free of scars, it's probably quite young. But nothing accurate that I know of without cutting the poor thing in half and counting the rings!