Thanks - some info:around 6 to 8 years.. really depends on many factors like temperature, water hardness, ph etc..
I checked literature for you, at 4 years it is better if you keep water a bit softer side so his body doesnt need to process much of harder water and food as much live food. other than that I cannt really judge based on general "bit off". maybe a photo to see his coloration, water information.
if you kept him at high temp. range for his whole life, he might have already lived his life to end.
Hopefully he gets well.
Sadly I think most of us are past our prime but still he is such a wonderful fellow - of all the fishes i've kept he has made the most impression - though in truth there are many species of fishes i've not kept nor will i ever keep.He, like me, has passed his prime, but we're not dead yet!
Will try to get a picture; the spine is just slightly bent as seems to happen with old fishes you probably wouldn't notice if you hadn't see him in his prime. It will be a little bit on the picture - I only seem to see him about once every 3 weeks right now - his 'home' is in an area of the tank that is not visible.tds and everything sounds well in range. Try increasing temp by 2 or 3° C slowly and give him maybe some discus high protein food for 2 times a week.
maybe he recovers but bent spine doesnt sound so much promising tbh.
generally indicates internal parasite or calcium deficiency or old age.
share a pic to see his coloration then maybe we can judge a bit better.