I think it depends what you do. If you're working in a factory on the production like that offers little mental stimulation, but if you get to engage the brain cells then that's different. My grandfather got bored after he retired and went back to work as a director of a car dealership. And look at Stephen Hawking - he's at an age where most people have retired but he's taking up a new position. A lot of scientists and people like that who are passionate about what they do wouldn't enjoy retirement.