the geneva convention, or some such organizaton, has a 'rule' about soldiers, -- 18 is considered the youngest for combat soldiers..
The US got in a bit of trouble not to long ago.. our armed forces, are purely volluntary, and the youngest age for enlistment is 17-- in theory (and in fact!) we have sent 17 year olds (ok, only a month or two shy of 18) into combat.. a no no!

Our standards of age and conduct are always changing.. why not that long ago, 18 was considered a legal age in US to buy a drink.. now its 21.. and some states allow kids as young as 14 to opperate a car!