The scales provide the best clue to telling the age of a fish.
Scaleless when born, fish grow scales under their outer layer of skin to provide waterproofing. As they age, most species add growth rings, that is, increase the size of their scales to cover their larger bodies.
With a very old fish, however, there are fewer additions, making it difficult to pinpoint its age.
This method also does not work for fish without scales, such as catfish.