Why are the youth “doomed”?

The youth in this poem are doomed because they have been called to a war—World War I—which will steal their childhood and innocence. In addition, these young boys are doomed to never truly grow into men, as they are fighting a war which will most likely take their lives. They will "die as cattle," only seen as one of the multitude of soldiers who have sacrificed their future to the hands of war.
At their passing on the battlefield, they will not even receive an honorable funeral or farewell. There will be no ceremonies at their death or speeches of their bravery. There will be no last prayers over their body or tender last moments with those whom they loved most. The only ceremony that they will get is the violent sounds of battle, such as the "monstrous anger of the guns." The only sounds of grief will be the jarring sounds of "wailing shells."
In this poem, Wilfred Owen uses these boys' tragic deaths to show the horror of war and expose just what this event steals from those who are called to participate in its brutality.