There are a few things that would be good to keep in mind about the military. They aren't all alike, they come from all over the place. It can be a bit of a culture shock at first. Another is that they are young. Your perspective on what counts as "old" can be different in the military. There are a lot of people in their 20's, and progressively less and less of older people. A 50 year-old can be seen as ancient. Think if it like professional sports, a 40 year-old NFL player is seen as ancient, but in a normal job, isn't even halfway to retirement.
One thing I liked about old WWII movies is that they depicted the military fairly accurately. The soldiers weren't supermen, they were just people. They didn't all think alike, the sterotype of the soldier from brooklyn, the soldier from Alabama, the soldier from Idaho works because that's pretty much what it is like.