From a retired Air Force Master Sergeant, here's the answer: It depends. The normal rules are that the junior never calls the senior by their first name. They are addressed by their rank or Sir/Ma'am in all but a few instances. The Navy and Coast Guard call their commanding officer "Captain" even if they do not hold the rank of O-6 or not (Navy and Coasties feel free to correct me). In some organizations/situations the rules are "bent". Aircrew and Spec Ops types are in a totally different culture than the rest of us slugs. In some instances, enlisted call officers by their first name (USAF aircrew that have enlisted and officer members). I was a regular guy when I ran into the "highly irregular" practice of first name addressing during my last assignment in the Air Force. I spoke with the senior member (Lt. Col.) about how a major called him by his first name. Basically, it is allowed in their circles for convenience of in-flight operations. I also spoke with some fellow enlisted guys about this. It was the same answer. However, I will say that they are cognizant of the rules when they are not around their "own kind". It is a bit of a juggling act.

Now when it comes to retired life, some people choose to call people of certain ranks by their former ranks out of either habit or respect. In my case, I have a habit of calling a retired Chief Master Sergeant (USAF highest enlisted rank) "Chief" (an acceptable in-service form of address). I do the same for full-bird Colonels (Air Force/Army/Marines)/Captain (Navy/Coast Guard) and Generals. In post-service situations there really is no need for the formality, it is just personal preference. Now, if military retirees are in uniform for an event, then the protocols are typically expected - Rank or Sir/Ma'am, saluting, walking positions and so forth. Finally, if someone calls me "Sergeant", I feel honored. Does that help?