Goat Rodeo Cowboy makes a good point. I know people who work in industry ( I am still a lab technician by trade and did radio on the weekends most of 30 years) who are in their 70's and still working. They like doing what they are doing and it gives them a sense of purpose, or if you will a reason to get up in the morning.
I know others who did retire from whatever type of work they did, and who have enough other interests and may volunteer in some capacity to keep active and be useful. Some have enough money that they travel and do things so that they truly have enjoyed their retirements or started a second career, possibly in something completely different than the work they did most of their lives.
I believe that those who keep active, both physically by doing things, even if at a slower pace, and those who keep their minds active, reading, writing, taking college classes, working, volunteering, etc tend to live longer than the rocking chair set, who've sort of given up and seem to lack a reason to get up in the morning.
I don't believe there is any evidence to support this next thought I have, but who knows, there even may be some folks who use talk radio as a vehicle for them to stay active in the political sense. Then by getting fired up about some issues or issues by Rush/Hannity/Beck, or on the lib side Air America, Bill Press, NPR, etc, they either stay involved or get involved possibly for the first time in their lives politically other than simply voting. It gives them a reason to get up in the morning trying to help make the nation or planet a better place. Far better than the seniors sitting around watching soap operas on TV all day and becoming vegetables. I can hear Rush now proclaiming that he's providing a public service to the seasoned citizens of this nation, by giving them a reason to live, etc.
We'll never know, but it's possible that by doing his radio shows all these years (obviously Mr. Harvey loves what he does) has kept both his body and mind active giving him a reason to live into his 90's.