How common is it for tenured professors to retire?
Under US law, since 1993, there is no mandatory retirement age for college faculty.  They can keep working as long as they are physically able to work, and many faculty are choosing to work into their 70's and 80's.  See this recent article from Inside Higher Education that gives some statistics:

https://www.insidehighered.com/news/2013/08/02/new-study-shows-difficulty-encouraging-professors-retire