I'd say that the only true way of knowing how old a snake is , is, if you have had it since a hatchiling....
You could judge his age with his size but the problem is each individual animal grows at their own rate depending on their appetite, hence some get huge really quickly and some will take a long time to grow... an other problem with this is that, snakes do not grow all their life. Once it has reached its full adult size - it will stay of the same size for the rest of his life or barelly grown.. In many species a 6 or 15yo snake will be of excact same size..
I guess if you buy an adult, you can always try to make a background check to try to figure out how old he or she is...
Hope it helps
1.3 Coastals 6.6 Jungles
3.4 West Papuan 1.0 Bred'ls
1.1 Yellow condas 0.1 Sebea
**looking for female Bredl's python**