It is known that about 250,000 boys served on the front line during World War I, whilst being under the age of 19. This was the official age at which overseas service was permitted. This BBC guide gives a useful introduction to some of the reasons for a conspiracy of silence around the enlistment of boys, which was especially prevalent before the introduction of conscription in 1916.
The youngest authenticated combatant of the First World War is Sidney Lewis, from Tooting, South London, who was able to join up at the age of 12 years and 5 months, and saw active service on the Somme for six weeks. A letter to the War Office from his mother demanding his return resulted in his being withdrawn from the front line, discharged from the Army, and sent home.
William Edward Shilvock Wright
In an article in the Birmingham Post 22nd November 1918 reporting the death of his eldest brother, Second Lieutenant John Shilvock Wright, it is mentioned that W. E. S. Wright served at Loos and on the Somme at the age of 15, returning home after being gassed, and then rejoining on attaining military age. He was training for a commission at the time of his brother’s death.