Fish are aged by reading scales or otoliths. When fish grow they leave rings on their scales much like tree rings. Otoliths are inner ear bones and they are often easier to read then scales. The problem is that to be able to read scales or otoliths you need both experience and a microscope.
If a fish is from a lake that does not have natural reproduction an age can often be guessed based on the stocking records.
Do you have a fish from a particular lake in mind?