My 76 y.o. father retired at 58, got bored and went back to work by 60. He opened his own estate planning law firm. Unfortunately, he needs my 76 y.o. mother to help him run it (she is administratively a whiz, with an amazing work ethic), and she'd just assume retire.
They are both slowing down a bit. The work has dried up enough to match their slow down. At least for my dad it appears to be optional. My siblings at times have speculated they are working because they need it. They are very secretive about their finances (as is their right), but they taught me to be a saver, I am pretty sure they practiced what they preached.