for which countries and at what time?
For example in Germany in 1945 it was not unheard of for some soliders to be 14. and Japan in 1945 had kamikaze pilots who were 16.
The same is said for Russia in 1941, 1942.
People were believing that if there was an invasion of japan there would be children under 10 fighting.
In general the armies wanted individuals who were over 18 and under 35.
they set up "home guards" which had the older men who were above 35, and youth brigades (think boyscouts) to prepare the younger teenagers to go and fight as well.
