Buying your veiltail betta from a reputable breeder is the only way of knowing how old your fish is. Many bettas sold in pet stores are around 6 months old. You may guess your betta's age by watching its behavior. Betta fish have an average lifespan of 2-5 years. They usually start slowing down after reaching three years of age. If your betta seems less active and sleeps more than it used to, your fish is likely entering old age.