Grave of Private John Condon
The youngest soldier to be killed in the Great War of 1914-18 was 2622 Private John Condon, of the 2nd Battalion Royal Irish Regiment and came from Waterford in Ireland. Like a quarter of a million other boy soldiers from all over Ireland and Great Britain, John was underage. He arrived at the Western Front in March 1915 and two short months later he was dead. Killed in a German Gas attack at a place called Mouse Trap Farm near Ypres, Belgium on the 24th of May, a day they say when a greenish yellow mist crept from the German lines with deadly poison. John’s grave is in Poelkapelle Military Cemetery (CWGC) and is now reputedly the most visited grave on the entire Western Front for obvious reasons. There, amongst thousands of white headstones, there is usually an array of poppies, flags and wooden crosses around the final resting place of young John Condon and a gravestone that says it all. 6322 Private John Condon 24th May 1915 Age 14.
CWGC memorial book at the cemetery
Michael Fisher's News 
The attack came at 2 am. Condon isn’t in that grave. It is a Private Fitzsimmons, also Condon was older than stated.
I presume this is the reasoning behind your comment:
Taken from the History Collection, The Tragic Story of Pte John Condon. Is It All a Lie?
The men that collected the body believed it was Condon because they saw 6322, Condon’s service number, on part of a boot. In Waterford, there is a birth certificate that clearly says a boy named John Condon was born in October 1895. This means the youthful adventurer told the truth about his age. In 1938, however, a local newspaper, The Waterford News, claimed that John Condon’s brother Patrick had stowed away, pretending to be his older brother. Patrick’s older cousin Nicholas was the source of that particular story.
Perhaps neither John or Patrick Condon is the body buried in Poelcapelle Cemetery. Condon’s Battalion never fought in the place where his body was allegedly found. The Royal Irish Rifles 2nd Battalion, on the other hand, did fight there. On June 16, 1915, Patrick Fitzsimmons, one of the Battalion’s members, was killed in action and had previously fought with the Rifles’ 4th Battalion. His service number? 6322. Fitzsimmons’ body was never recovered or identified.
There is a certain level of ambiguity associated with the stamp found on the supposed boot of Condon. It said 6322 4/R.I.R which was taken to mean service number 6322, 4th Battalion, Royal Irish Regiment. This is how the Imperial War Graves Commission interpreted the identification. The trouble is, Condon was apparently in the 3rd Battalion. R.I.R could just as easily stand for Royal Irish Rifles. Although no one can offer conclusive evidence, there is more than an element of doubt over Condon’s age and identity.
See: https://historycollection.com/tragic-story-private-john-condon-youngest-soldier-die-world-war/2/
Almost certainly Fitzsimmons body, also, evidence points to a burial nearer to Bellewaarde than Poelkaepelle.