In the military, you generally have to serve for at least 20 years to retire. You can also be medically retired with less years in service if you have medical issues that prevent you from continuing service. If you are not retired in one of these two ways, then you're a veteran once you leave the service. It is not considered being retired from the military.
Umm, not really. Every Marine I know (and I am a Marine, so I know a lot of them) will say, "I am a Marine" not "I'm a former Marine." It's a Marine Corps thing and has nothing to do with job title, time served, or how many times someone got blown up. It's about the brotherhood, the comraderie, and a way of life that does not end just because your enlistment is up.