I've been wondering about this recently - if people box on too late, everyone is praying for them to quit.
When skaters slow down, they've gone soft.
Most boxers love their game first of all and many stay involved after they retire, but it is only for the headstrong at the rough end and only for a short window.
Not that many people skate big rails just for the thrill of it.
Some do and God bless them, but there is a lot of misery involved so the answer I would have thought is 'until it is not part of the job'.
In both fields, retirement tells you when its time and if you are smart you listen to it.