There is an interesting article here looking at the logistics of underage recruitsHow did Britain let 250,000 underage soldiers fight in WW1?
At the outbreak of war in 1914, the British Army had 700,000 available men. Germany’s wartime army was over 3.7 million. When a campaign for volunteers was launched, thousands answered the call to fight. Among them were 250,000 boys and young men under the age of 19, the legal limit for armed service overseas.