Longevity is a good topic for Rays and any other fish for that matter.If you consider generally the bigger the fish the longer it lives,for instance small tetras Barbs,Rasboras etc,active fish with fast metabolic rates usually live between 2 and 4 years where as Angel fish,Severums ang Geos regularly make 10 years plus.This brings me to Rays,most make reasonable sizes,i would have thought 25 years plus an easy age to attain for such fish,Redtail cats,Osphronemus gouramis and many other tank busters live this amount of time if properly cared for.........the Monks have Carp swimming around that are 100 years plus old......something to think about!