The "iron youth" is the manner through which the war is presented to young people in Germany.
Part of what makes All Quiet on the Western Front such a timeless read is that it probes into why wars take place. It analyzes how wars are "sold" to young people who willingly enlist, fight, and many times, die. Given war's brutality, there have to be ways in which in people would feel enthusiastic towards it.
The Iron Youth is one example of how the war was "sold" to young people in Germany. The Iron Youth" is a symbol of the strong German man. It was part of the campaign to enlist young people to fight for the German cause. It was rooted in nationalism, or excessive pride in one's nation. To be a member was representative of the youth that would defend Germany, leading it to military victory in World War I. The Iron Youth was a means through which school leaders like Kantorek to generate enthusiasm amongst their students.
In Chapter 1, one of the soldiers, Kropp, receives a letter from Kantorek calling soldiers like himself members of the "Iron Youth." Given how he has been exposed to the horrors of war, he repudiates with, "Iron Youth! Youth! We are none of us more than twenty years old. But young? Youth? That is long ago. We are old folk." The symbol of the "Iron Youth" was a way to explain the war to young people. It was similar to a form of propaganda to romanticize the war, making it better than it actually is.