The average age of a us infantryman in ww2 was 26 I believe. In Vietnam it was 19 and was kinda noted for being too young. Also in Napoleons grand army the conscription age was 20-25 so long as you weren’t married. Then when you consider special forces guys are all pretty much early to mid 30’s you kinda realize 18-20 year olds in the military are pretty much cannon fodder. A military needs “young” but capable (read life experience) to actually be effective. For example Russia recently upped their minimum age to 21 for that reason pretty much. Especially now that we are definitely in an age where technology is playing a major role in combat.