Corrected entry: Jim doesn't age during the movie. He was in the prison camp for about three and a half years. When we first see him he looks about 12. By the end of the movie he should be a gangly teenager but he hasn't aged a day.
Corrected entry: Jamie is captured and put in a prison camp in early 1942, yet he has no problem recognizing and knowing the name of the P-51 Mustang plane, which was only even introduced into USAF later that year, and didn't arrive in China until 1943-44.
Correction: The Mustang entered service with the RAF in 1941. It was originally produced for the RAF, not the USAAF, to an Air Ministry order in 1940. The USAF didn't become the USAF until 1947. Throughout WW2 it was the USAAF, a branch of the US Army.
Correction: While this is true, some license should be allowed given the awkward age range involved. Twelve to sixteen isn't really a big enough change to warrant a second, older actor, especially since many sixteen-year-olds don't look particularly adult. And there's not much you can do with makeup in that age range either. (It's not like he ages from five to fifteen, or twenty to fifty.) This is just a case of suspending your disbelief. Or, poor treatment in a prison camp could stunt his growth and/or delay onset of puberty.
Krista