As bettas get older, they slow down, like everyone else. Bettas can be expected to live till about two years of age. Considering the average age that a betta is sold is 5 months, that means you should have about a year and a half with each betta.
When the betta gets 14-15 months and older, he will probably sleep a lot more, be slower to flare, and generally not be the same betta that he was when you first got him. To make him more comfortable, heat and filtration can be added. A betta prefers a temperature of 78 degrees, and cold will make any betta slow down. Filtration decrease the number of water changes required, which reduces his stress.
Feedings may have to be changed. Watch your betta, and if he isn’t eating as much, don’t feed him as much. Feeding him extra fod just means that it will decay in his tank.
Be extra vigilent about diseases. An aging betta is more supsceptible than a young rapscallion. The best way to prevent disease is to keep him in a heated tank and to have immaculate water.
If your betta is in a community tank, he or she may need to be removed. If your betta is being harrassed or seems distressed from being in the tank, remove the betta and give it a well-deserved rest by itself.
Finally, when your betta passes on, remove him or her from the water promptly and drain the water from the tank if the betta was the only fish in it. Many persons let the water remain in the tank until they get a new betta, but it is more sanitary to wash and dry the tank, and let it stay dry until you find a new betta to love.