On rattlesnakes, you can supposedly count the rattles, although I think even that is dependent more on size than on age. So a younger snake that grew faster because it found more to eat may have more rattles than an older but smaller one.

As has been pointed out, there really is no way to know with a BP. Too much depends on individual tendencies, how much & how often it was fed, illnesses that may have affected its growth, etc.

If you take the snake to a vet that tells you how old it is, it is more likely that the vet either doesn't know enough to know he can't make a reliable estimate, or he is just giving his best estimate in order to make the customer happy, even though he knows there is no way to be sure.