To answer this question directly, the average age from my research is 69 years old. However, in my personal experience I've seen some practice into their 70s. I will say that retirement isn't always about how much money one makes, because dentists make a lot of money compared to the average American salary. However, with how much time and effort dentists spend to become licensed they don't always want to retire. They enjoy what they do, and I think this applies to several fields of work.
Hope this helps.