My grandfather, who lived in Northern Ireland at the time, ran away from home and joined the army in 1914, when he was 14, using one of his older brothers’ birth certificates. According to my mother, he said one of the happiest days of his life was when his mother turned up at the training camp, with *his* birth certificate, to retrieve him!
He knew there was going be a war so he did the enlistment tour in 1940, got turned down by all, to many foot ball injuries. Enlisted in the Coasties, hey the Great Salt Lake has a station, spent the war casing Germans.