At the end of WWII, untold thousands of German soldiers were either very young (<16) or very old. They were combat troops and they died in the thousands. That was tragic because they weren't up to the physical demands of combat, were poorly trained, equipped, and led, and were little more than cannon fodder.

This doctor is an oral surgeon. He won't be humping a pack or engaging in firefights. He'll be applying a lifetime of skills to help wounded soldiers. It isn't age per se that's the issue, it's the type of duty they're pulling. Some military jobs aren't physically demanding at all while others are extremely rugged.

There have been several people in the military who were pretty old when they retired. For example, Adm. Grace Hopper served in the Navy until she was 80 years old. Many of the great WWII generation of generals and admirals served well into their 60s, and would likely have stayed in service longer had it not been for the massive reductions in the military after the war. Nimitz retired at age 62. Omar Bradley retired at 60. One of my neighbors was on active duty in the Air Force for 44 years as an enlisted man.

There are also people like this who want to serve. More power to them.