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.
Definitely they can decide for themselves since it is their body. Only the owner of the body can decide for himself even though it is obvious that retirement is due.
Just like what goinmerry said, there would be no interested organizers or other parties to make the fight happen even though these fighters still wanted to fight because they are afraid that the fight will not be a blockbuster nor will be profitable for them. But if at any point they will be granted a fight because there's some interested to make the fight happen then I guess by that we will see if for example Nonito Donaire made the right choice instead of retiring.