Circadian disruption is known to be bad for your health, and a career of that will most likely take years off of your life (if something else doesn't get you first).

But flying night freight after age 65 is probably not going to matter much, because your material condition at that point depends much more on what you did for the last 45 years, and less on what you're going to do tonight. Also presumably as a semi-retiree without kids at home, you have some lifestyle flexibility and can stay mostly on the back side on your days off. The back side in and of itself isn't that bad, it's the flopping back and forth that hurts.