Average life expectancy is a notoriously bad measurement because it skews way down because of childhood deaths.

For example, Roman life expectancy was only 21, but if you made it to age 5 it jumped to 42.

Even then the laws at the time was 18 to enlist -- so they already viewed child soilders the same way we do now. The big difference is age verification was a lot harder because there was little in the way of a paper trail.