last time I looked it up i understood the rules to be as followed.
17.5 years of age, could sign up if mom or dad signed for you. when I joined the res. there was like 25 guys who joined with me, but i was the only one over 18 who could sign for himself.
I believe the thinking was that it would take the 6 months to train the average soldier and prepare him to go over seas during ww 1 and ww 2 and he or she would be 18 when the ship sailed.
There were a lot of underage soldiers on the front lines who were not caught in the system.
my great uncle joined the merchant marines and sailed the north Atlantic untill he was old enough to be sworn into the RCAF ( his father was the head recruiting officer for Ontario. My great grand father was shocked to find out he was called to swear in his own son, he did not know he was home from sea) My great uncle became a gunner on bombers that flew out RAF Kinloss in Scotland, died in a training flight just after his 18 birthday.
on my dad's side of the family most of his uncles ran away to join up, alot of guys from my home town tried to enlist in the local units but were turned away because of underage so they went to eastern Canada or Toronto and signed up there where the fact that they were underage was not known.
great uncle turned 17 in korean, was with the RCR and sent back went back after he was 18 for a full tour so to speak.
so under age does happen, and 17.5 is the starting age