Long trailing fins imbued with rich colors of royal purples and blues give the betta fish an air of elegance. Under ideal conditions, a betta fish can live for up to three years in captivity. You can approximate the age of your betta to some extent, but several factors can make it nearly impossible to discern a betta's true age.
If your betta fish is on the small side, chances are good you have a juvenile. Adult bettas average 2 to 3 inches in length -- that's a measure of the main trunk of the body, not including the fins or tail.
Long, flowing fins are a good tip-off that your betta is a male more than a year old. Only males have the long fins. Juveniles and females possess shorter fins.
Older betta fish will usually have signs of wear and tear -- such as ragged edges on the fins, lost scales and white spots on the skin. Older bettas' backs may become humped; they may miss when they strike at food. Older bettas may spend more time resting in the plants rather than swimming actively.