The average lifespan of a betta is 2 to 5 years. As the betta ages, he slows down and becomes lethargic. You might notice he is becoming sick more often with diseases such as ick or fin rot, poor eyesight or muted coloration. An aggressive betta who used to flare at himself in the mirror may stop putting on that impressive show. As his age progresses, the betta will frequently lie on his side either on a plant or at the bottom of the tank. They are very vulnerable at this stage of their lives, so it's imperative you keep up with water changes, watching out for opportunistic tankmates and anything that may stress your betta.