Desire isn't enough to win a fight. When the mind is still active but the body is lagging, you know it is time to retire. We can observe it in the fight too. As spectators, we are keener to changes than the ones who own the body. He might feel he is still in optimum condition but if the reflexes deny him then it is time to retire.
Then let these boxers experience several defeats until such time that realization will come that they need to stop for good. We can't just ask them to retire if they really want to push through their careers even if they are already at their considered retirement period. Obviously, we are not in place to give them recommendations especially if their mindset is really set to still fight.
Don't worry bro. Nonito Donaire knows when he will stop. It's not that he will really push and aim for something if he thinks he can't do it.
The time will come that he will finally announce his retirement but only he knows when.