How would I notice old age in my betta?
Firstly, when anything is old, it usually will slow down as the first sign of aging, and for a betta this means that it will not swim around as happily as it once used to, and you may notice an increased amount of resting in between swimming periods, and longer sleeping amounts. Another sign that you may be able to notice is that your betta has a curved spine where it meets the head, which also may be flattened on the top as well. Some of the common disabilities that come with betta aging are trouble reaching the surface in the tank, struggling to swim as easily, and eyesight. They may also not respond to the same food you have been giving it, as they often lose their keen sense of smell when they age, and rotating foods might be necessary.
What do I do if I notice these symptoms?
If you notice that your betta isn't swimming up to the surface very easily, then you may want to lower the water level to make it easier or switch to a smaller tank with less height to help accompany this issue. For an issue with an eagerness of the appetite, then you may need to remove freeze dried and pellet or flake food from their diet and switch to frozen foods, as they have a very potent smell and don't pose as much risk of causing bloat than flakes or pellets would. Be sure to always soak the food before you feed it to your betta, thaw out frozen foods until the cube has completely separated.
You can still use pellets or flakes during this time if you don't notice any issues with digestion, but I personally would recommend to use a very small sized pellet, and crush up flakes to smaller bits to prevent the betta from gulping in extra air when trying to consume the food. Soaking the food will cause it to expand in the water, and not in the stomach of your betta, this is a huge cause of overfeeding and bloat in other bettas.
Is there anything else that I can do for it?
If the betta still has some mobility, meaning it can still swim around on a daily basis and eat properly, then it may be just fine to continue living as it always has, but the keeper takes note that it is weakened and must be paid extra attention to in case of a problem that may arise at any time, and there is a way to overcome that issue or deal with it accordingly. Take note that old age increases the risk of contracting any type of disease much easier, and extra caution should be taken. The only sad part of this situation would be that the betta's life may not continue for much longer after a certain point, and if the fish is in any sort of pain or complications that are dangerous to its life then it should be properly dispatched to humanely prevent it from suffering any longer.
This is an example of an old betta, he was rescued from the previous owner who had him in less than ideal conditions.
He came to me with bad bloating and decreased activity due to not eating, and eventually, he was cured quite well and lived a good life after that.
Here's another example of an elderly betta still in our care, you can notice how long and ragged the fins are becoming.