Well, since one of our cats is in his 20s and is just starting to act like an old man I would say I wait for the cat to tell me when they're a senior
I like this answer…. My childhood cat, Rascal, lived to 19, she was an outdoor/indoor cat who never used a litter box and slept in my bed every night until I moved out… I miss her still, then cats lived forever even if they were outside as well, now, here and in my hometown, you let them out, they rarely make past 5 or 6 before a coyote gets them…
I think one side effect of indoor only cats though, is so many overweight cats… They just don't get the same activity level and if the owners are not very careful in their food and diet.. They can get chubby faster, I know mine did at first, I had always had cats that were allowed out.. I lost too many, Bolts was not truly *allowed* out but he wanted out so bad he'd escape every now and then… One time he did not come back.. I no longer let them out and guard my door like a maniac even though I feel mean at times, and Nuts was packing on weight fast.. I had to adjust for his being inside more…
Nuts is 6, Mini 4 and they both still act like kittens!!