What was the average age of a soldier?

Answer 1

Presuming you mean an enlisted US soldier serving during the Vietnam War (1960-1970, approximately), then there are two answers:

Prior to the mass-introduction of US ground combat troops (i.e. the so-called "advisory" period of US involvement, roughly 1960-1965), the majority of soldiers were still volunteers, and many were professional career soliders. As such, enlisted would have an average age around 22, with NCOs averaging 27.

With the introduction of US ground forces for combat, the enlisted ranks were generally filled with draftees. Over 90% of all draftees were in the 18-20 range. By the time they were drafted and had undergone training, then deployed to Vietnam, the average age of a enlisted soldier was a bit over 20; the age of NCOs had also shifted downward, as many of the former volunteer force were promoted to fill new NCO positions, so the average NCO was now almost 25.

Thus, prior to 1965, the average age of an infantry platoon (25+ enlisted, 3-4 NCOs, and 1 junior lieutenant) would have been about 24. Post 1965, it would have been 21 or perhaps 22.