Are you saying it is impossible for a person to change their stance based on time and increased exposure to something? Heck if you had asked me when I was 20 my stance on any number of things and then asked again today, you'd find they had changed. The basic facts about them haven't changed, but my perspective and exposure to their consequences sure has,and therefore so has my understanding and attitude towards them.

How is it so unbelievable that a kid wanting to serve his country was changed by the experience of doing so, having been placed to see what it means firsthand?

Finally, it is well understood that brain development continues into the mid 20s - so changes to thinking should be physiologically expected at least.