When the swim muscles atrophy and are too far-gone to recuperate. Each body is different, but by 90, it is a rare specimen that can swim. It is natural for a ninety year old (forget those darned hyphens) to swim quite well, but the average 90 year old has not kept up any exercise routine and more than likely is unfortunately either dead from lack of proper self-care (no proper nutrients, no exercise) or infirm with brittle bones, no muscle to speak of but atrophied strings and no strength, and skin like tissue paper. The water alone needs to be tepid and applied with care. This is not a swimmer. But if that same individual were to eat right and exercise and swim?
They could swim well into their 90's and even beyond, I guess.
Total screw up on my part---deleted wrong part of my answer; it was a reference to the destructive nature of the testing that you would probably be conducting.
Unfortunately, the subjects necessary to participate in tests that could provide some data on which age is in fact too old are unlikely to volunteer.
WM BARR . =ABSOLUTE TRASH 