How can you tell how old a fish is?

You can tell an age of a fish by the scales. Fish do not have
scales when they are born: scales grow under their outer layer of
skin in order to provide waterproofing. As fish age, most
species add growth rings - which is increasing the size of their
scales to cover their bodies as they become larger.

With older fish, however, there are fewer additions which makes
it difficult to determine their age. This method does not work
for fish that do not have scales, such as the catfish.