How do you tell the age of a Neon fish?

By its general size and appearance. Most Tetras are not long lived fish and depending upon species, live for 3 to 5 years. I would advise buyers to always buy from a reputable breeder if possible. If you can't find a good breeder then use a reputable pet store and after researching the species, buy fish from healthy looking, active schools of tetras that are not quite full size.