I'm not Air Force, just retired Navy.
I have a character who was a U.S. Air Force officer, worked in Intelligence or cybersecurity. He's well connected, closely allied with a prominent senator. Then he retired to start his own civilian defense contractor business along the same lines of cyber-intelligence. (story set in near-to-mid future)
Premise sounds good -- I've known a few like that.
Questions:
What would be an appropriate rank for him? Colonel? At what age could he retire?
Colonel would be best -- gives him enough time to build up those contacts, and is senior enough for the senator to give him the time of day.
Active service from 24 to 30 years. Age would be from 45 to 55, depending on accession source (Academy, Officer Training School (OTS), Airman Education and Commissioning Program (AECP), Reserve Officer Training Corps (ROTC), etc.)
Commissioning source can have a wide affect on relationships between and among officers and senior civilians. Academy grads are considered more 'Old School', while AECP grads are more mavericks.
As stated, your MC is likely Air Force Academy.
Can a civilian contractor business be located on the grounds of an Air Force base?
As DOD office space temporarily assigned to the winner of a service/support contract? Yes. As a stand-alone office of the contracting company? No.
If, say, the base commander visits him, how would the base commander address this retired officer? Still call him Colonel (or whatever appropriate rank)?
"Howdy Rich, how's your bride? And your daughter, Sally? She's married herself now isn't she?"
"Mr. Burkett, your employees refuse to show proper courtesy to my staff! Fix it, or I'll fix it -- fix you right off of this base!"
As another poster mentioned, a base commander is flag rank. He's (or she -- near future makes it possible) not going to call a retired officer by their retired rank. As a matter of fact, the US Air Force is big on using first names in situations where the other services would use ranks.
-- Duncan