Yes, it depends on the individual. Irving Crane won his last major tournament at age 72, IIRC. I remember him playing Luther Lassiter on TV years ago, Alan Hopkins was commentating. It was clear that Luther was well past his prime, but Crane was playing very solid...still a AA player for sure and he was torturing Lassiter. Hopkins commented at the time that Crane was still feared by everyone in virtually any tournament he entered, however the other HOF players in the exhibition had long since hung up the cue. It wasn't even close to fair.
To answer the OP, it seems that most world class players hit a health wall in their 50s...their eyesight starts to go, then the joints and back make it very hard to play at their old speed. You can start to see it in Efren now, Jose Parica started sliding a few years ago. Obviously, they're both still feared everywhere, but not nearly as dangerous or dominating as they were 10 or 15 years ago. Sure, there are exceptions to every rule; but it seems like the magic number is somewhere in the mid 50s.