The dictionary definition of a subaltern is a junior army officer below the rank of captain. In other words a First or Second Lieutenant. However, in the context of the First World War that does not capture the full meaning of the word.
In his book, 'Six Weeks - the Short and Gallant Life of the British Officer in the First World War', John Lewis-Stempel admitted how much he had come to admire these young subalterns during the time he spent researching his book and quoted one former soldier, Private AM Burrage, who wrote, "I who was a private, and a bad one at that, freely own that it was the British subaltern who won the war" [War is War by Ex-Pte X Gollanz 1930]. So who were these subalterns and how did they 'win' the war?
In the early days of the war many young officers were volunteers or territorial soldiers, in other words not professional soldiers, and almost all of them had been to public or grammar schools. RC Sherriff, author of Journey's end, claimed that early in the war you had to have been to a public school in order to qualify for a commission, saying that he himself was turned down because he had been to a grammar school. But I have heard Gary Sheffield say that it wasn't the fact of the public school that mattered but whether or not your school had had an OTC of which you had been a member that counted. Public schools were much more likely to have had an OTC pre-1914 than many grammar schools.
But to Sherriff:

"these young men never turned into officers of the old traditional type. By hard experience they became leaders is a totally different way and, through their patience and courage and endurance, carried the Army to victory after the generals had brought it within a hairsbreadth of defeat". [The English Public Schools in the War, RC Sherriff in Promise of Greatness ed. George A Panichas, Cassell 1968].

Later in the same article Sherriff wrote:

"Without raising the public school boy officers onto a pedestal it can be said with certainty that it was they who played the vital part in keeping the men good-humoured and obedient in the face of their interminable ill treatment and well-nigh insufferable ordeals".

Unlike junior officers in the German army, British subalterns lived with their men in the trenches, cared for them, shared their hardships, led them into battle and died with them. As EA Mackintosh says in his extremely powerful poem, In Memoriam, inspired by the letter of condolence he was writing to the father of one of his soldiers killed in the recent fighting, :"You were only David's father but I had fifty sons". Aware of the responsibility he had for them, Mackintosh writes:

Oh, never will I forget you,
My men that trusted me,
More my sons than your fathers'
For they could only see
The little helpless babies
And the young men in their pride
They could not see you dying
And hold you while you died.

Happy and young and gallant,
They saw their first-born go,
But not the strong limbs broken
And the beautiful men brought low,
The piteous writhing bodies,
They screamed "Don't leave me sir",
For they were only your fathers
But I was your officer.

"My men that trusted me" - there's a lovely letter quoted in Laurence Housman's War Letters of Fallen Englishmen from Lieutenant HM Butterworth, which illustrates this trust beautifully:

"... no digging or wiring party party ever goes out without an officer, that is the way to get the men along. If one takes out a party of men somewhere they don't know - in the open probably - to dig, they'll go like lambs as long as they've got an officer with them. The curious thing is that in civilian life they've probably cursed us as plutocrats, out here they fairly look to us. The other night some time ago, I had some men and had to get somewhere I'd never been before in --; as a matter of fact it wasn't difficult and we had ample directions so before we started I was told to send the men with a sergeant. Said the sergeant to me, 'I wish you were coming sir, I don't know the way.' I said, 'My dear man, nor do I.'To which he made this astounding reply, 'Very likely not, sir, but the men will think you do and they know I don't'."

In a deferential age the soldiers expected their officers to come from a higher social class. But as Sherriff concluded, this didn't mean they were toffs:

"It had nothing to do with wealth or privilege. Very few of the public school boys came from the landed gentry or distinguished families. For the most part they came from modest homes, the sons of local lawyers, doctors and schoolmasters - hardworking professional men."

This was just the class that William Steele Young came from. His father, Archibald Young, was a cutter and surgical instrument maker. William and his elder brother, Archibald, were educated at George Watson College a fee-paying day school in Edinburgh. Archibald, a territorial soldier serving with the Royal Scots, was mobilised on the outbreak of war. William, studying engineering at Edinburgh University and a member of the University OTC, volunteered and was gazetted second lieutenant on 1 September 1914. Archibald was killed in action in Gallipoli at Saghir Dere on or about 28 June 1915. He is commemorated on the Helles Memorial. William, who also served in Gallipoli and then Egypt and Palestine, was killed in action on 2 November 1917 during the Third Battle of Gaza.
Arthur Young was proud of both his sons, proud that they had both been members of that "incomparable brotherhood the British subaltern".