Veteran Marines
Marines and those familiar with Marine Corps tradition will often object to the use of the term "ex-Marine" or "former Marine" because Marines are inculcated with the ethos "Once a Marine, always a Marine". The terms "former" or "ex" refer to something that once was, but is no longer as Col Wesley L. Fox, USMC (Ret.) states in the welcoming theater video at the National Museum of the Marine Corps.
Veteran Marine refers to an individual that completed their service and has received an honorable or general discharge from the service (visit [1] and see Veteran Marine). Marines who have retired are commonly called "retired Marines". However, addressing any veteran Marine, "Marine", which they still are, is appropriate. Veteran Marines may be addressed as "Sir" or "Ma'am" out of respect, or, according to the "Commandant's White letters" from Commandant General Gray until present, by their earned rank.[50]
Marines who have disgraced the Corps and Marines who receive a dishonorable discharge are referred to as "ex-Marines" or "former Marines".
Strange, where did you pull that from? I'd be hesistant to use such a term such as veteran due to the literal meaning of the word. I've been taught to use former as well by veteran/former Marines....
The fail safe would to just address the man or woman as a Marine, regardless of age. OORAH!