Every betta ages on his own schedule, so the useful comparison is never with another fish or with a photograph from the internet. Bettas often live around two to four years, and the practical question in those later years is whether today's change grew slowly out of the fish you know, or arrived as something new. Baseline, speed and company answer it together.
Building his baseline while he is well
Note what normal looks like for this fish, in writing, before you need it. Record how he greets your approach, how quickly he finds food, where he sleeps, how long his rests run, how his fins sit at ease and how his breathing looks at rest. A few lines a month keep that baseline honest, because memory quietly rewrites a slow decline as sudden or a sudden change as long-standing. Photograph him monthly in the same light, and file the shots in date order beside the notes. With a baseline held this way, a real change stands out within days instead of surfacing after weeks of drift. Note the tank's water results on the same lines during any week when food, decor or tank mates change, so later comparisons stay honest about what else was happening.
Normal old age: gradual, even and still himself
Ageing spreads itself thinly across weeks and months. Swims shorten, rests on leaves lengthen, colours soften, and the turn toward food loses its old urgency, yet the essentials hold: he still eats at each feed, breathes evenly, keeps his fins clean and intact, and answers your approach with interest. A curved posture at rest and eyes that look a little cloudy can also belong to age in some older bettas, provided appetite and breathing stay steady and nothing new gathers around them. The pattern to trust is slowness without loss. He does everything he always did, at a pace that keeps dropping gently.
Signs he is unwell: fast, gathered or new
Trouble usually breaks the pattern in one of three ways. It arrives fast, over days rather than months. It gathers, so refused food appears beside clamped fins, rapid or laboured breathing, a thinning body or new marks, instead of arriving alone. Or it introduces something the baseline never held: resting on his side, floating oddly, gasping at the surface, or hiding in a fish who always rested in the open. Any single new sign deserves a water test and close watching, while a gathered cluster deserves prompt advice from an aquatic vet. Colour loss, appetite loss and lethargy can each reflect several causes a vet may investigate, so describe what you see rather than naming what you fear.
Test the water first, then decide
Poor conditions can mimic illness closely in an old fish, since age lowers the margin he has for coping. Test ammonia, nitrite, nitrate and temperature on the day you notice a change, and correct anything adrift in gentle steps. Keep his layout and lighting familiar while you watch, feed small amounts he can reach easily near his rests, and remove uneaten food promptly so the water stays clean for a fish with little reserve. If he steadies as the water settles, keep the notes and the new routine. If signs persist in good water, gather rather than ease, or include breathing trouble or side-resting at any point, contact an aquatic vet and describe the timeline.
The dated baseline inside KinsLe's Pet Passport gives that vet conversation a beginning, a speed and a pattern, which is what separates an old fish from an unwell one.