Your thought on John Wayne in this movie?

That's why it's a movie and not a documentary.

Indeed, the average age of troops in Vietnam was 19. But that doesn't mean that there were no older soldiers. In almost every conflict of the last century, the senior officers in theatre tend to be far older than the troops on the front line.

Would the older ones have been involved in combat ops. as portrayed in the film? For the most part, probably not. But then, would the movie have been made without John Wayne in the lead?

Most of them were bachelors or had no children(Fonda being an exception).

One ought to consider each individual as an individual. On December 8, 1941 when President Roosevelt requested a Declaration of War against Japan that was soon returned the people that you list were in various stages of their lives:

John Wayne was 34-years old, married with four children was the sole source of support to his family. A deferment in such a case would be granted in a heartbeat by any draft board with any heart. He had no military experience and no military skills. Had he enlisted it is reasonable to conjecture that the army would have put him in film production as the did with most actors at that time. His star was rising and the war could benefit his career greatly, while entering military service would do nothing for the military that several million others (16 and a half million during the course of the war) could not do as well or better.

Jimmy Stewart was 33-years old, single and childless. His father was living and he had living brothers and sisters. He was about to be nominated and then to win the Best Supporting Actor Oscar for "The Philadelphia Story," but he didn't know that. He was also a trained and experienced pilot, so he campaigned to be accepted into the US Army Air Forces and after he was accepted he campaigned to be assigned to combat duty. He won that one too, and remained in the Air Force Reserve until he retired in 1964.

Henry Fonda was 39-years old, married and the father of two. He enlisted in the Navy and was packing cargo until the Navy ran him through OCS and put him into Air Intelligence (briefing and interrogating fliers, I would conjecture) for two years. It doesn't look like he did a lot compared to General Stewart, but he certainly did more than Mr. Wayne.

Clark Gable was 40-years old, on his third marriage, but had no acknowledged children. Apparently Loretta Young had a child named Judy who had been fathered by Major Gable during an affair in 1934. Neither parent ever publicly acknowledged the child's father. Clark Gable enlisted in the Army Air Forces, selected for OCS, commissioned as a lieutenant and rapidly promoted to captain. It's interesting how the military tends to do that, commission and promote actors apparently because they are actors. I don't include General Stewart in that group for the obvious reason that he earned everything he got. Anyway, Major Gable flew five combat missions in as many months primarily focused on making a recruiting movie for aerial gunners. I don't think the movie included a disclaimer that Major Gable was the only commissioned aerial gunner in the Army Air Forces. Still, he got shot at and flying in those bombers was dangerous business.

Eddie Albert had just started his movie career and had not progressed beyond bit parts. He was 35-years old, single and with no children. He enlisted into the Navy in 1942 and received a direct commission in 1943. He had a degree in business from the University of Minnesota, so I think the Navy commissioned him on that basis. He earned the Bronze Star with "V" device for his heroism during the Battle of Tarawa.

Douglas Fairbanks Jr. was 32-years old and married. He had three children with his second wife, but it is not clear whether any of them had been born by the time that war was declared. They had been married for about 30 months at that point, so it is possible, but not probable that all three children had been born. It is more probable that at least one had been born. His wife may have had independent means as she had been formerly married to a grocery tycoon, but his situation may have been closest to that of John Wayne's. At the time that war was declared Captain Fairbanks was serving as Special Envoy to South America, having been appointed to that position by President Roosevelt. It is not surprising that he was commissioned directly into the US Navy and assigned to work as a naval commando under Lord Louis Mountbatten. He remained a reserve naval officer until retiring in 1954 with the rank of Captain.

Whether you want to disregard John Wayne's acting because of his lack of military experience or honor him as a hero for portraying heroes on the screen is a personal choice. I choose to do neither. I will honor the memory of John Wayne as a great movie personality and for displaying American soldiers and sailors heroically.