At certain age, if you yourself will know its time. Its not like other career that doesn't need agility of your youth. Your skills doesn't depend on how fast you chop the spices but how good the taste of the recipe.
For us speculators, it's easy to say that they should just retire. We can't feel their desire to continue.
As for me, we should just let them do what they want. If at some point they experienced terrible losses and they feel their body is not cooperating now, they will retire right away without any hesitations. And even for let's say they want to continue, the promoters or organizers might not be interested now to form a fight and that will lead that old boxer to consider retiring and call it a career.
Yes mate, of course they have their own reasons why they still choose to fight even though they are not that young anymore and even though most of the fans wanted them to retire because of the age factor, part of those reasons why they still continue is that they believe that they can still deliver a win in the fights they participate because the power is still their and somehow they are agile enough to keep up with their foes.