My paternal grandfather was born in 1902 (I have a copy of his birth record). When he enlisted to fight for Canada in 1916, he said his birth year was 1898 (I found a copy of his WWI Canadian Attestation Record). According to my grandfather, when the people in charge found out about his age, he was shipped home. I don't have any record of him returning to Canada until 1918. I did a bit more research, and discovered that some who were underage were sent to training camps in England until they were 18 or the war was over - whichever came first.