The soldiers are “knock-kneed and coughing like hags” because the conditions of war are so terribly hard on their bodies. Remember that soldiers would typically be young men, especially during World War I, likely just eighteen or nineteen years of age. This is why the speaker describes them as “children ardent for some desperate glory.” Despite their youth and their physical strength, these young men are “bent double” like old, decrepit people burdened by a heavy load. Their knees are weak from “trudg[ing]” through mud, some of them even without boots, and they cough like very poor, old women. They have likely inhaled all manner of dust and debris from the explosions of bombs, and their health would be very bad from lack of sleep, bad hygienic conditions, and less-than-nutritious food at the front.
Descriptions like this one help the speaker to support his claim, that it is, in fact, not at all sweet and becoming to die for one’s country. He depicts war as something that breaks down the health and spirits of even the healthiest young people. It degrades them, ages them, and ruins their physical and mental health. It is not, as some people would have us believe, glorious and sweet; rather, it is tragic and horrible.