The only way to prove an animals age, is to have the orginal copy of a registration paper, Most breeds ask that the papers be returned when notified of an animals death. Also most breeds have an age that they consider the animal deceased by, I think it is age 25 in APHA and TB. I don't recall exactly
And after about age 30 its not possible to judge by the teeth .. ;)
Just because you have registration papers does not mean that they match the animal that they supposedly go with. You could have a 20 year old animal that dies, an unregistered 10 year old animal, and just say that the 10 year old is the 20 year old. Sure most breeds do DNA now, but 70 years ago, it didn't happen.