nowhere did I see any images of adults between the ages of 18 and 35.

They are the ones who have grown up with it and have known little else.

If you read personal stories of soldiers in WW1 you come across some who have spent two or more years in the trenches having been in their late teens at the start and when the Armistice came had wondered what they were going to do with their life now the war was over, they knew nothing else but fighting.