Explain lines 11-12 in to "An Athlete Dying Young."

In these two lines, Housman is using both allusion and metaphor to describe the nature of early athleticism. In referring to the "laurel," he is alluding to the practice in the Classical world of decking athletes in laurel wreaths to indicate success. In stating that the laurel grows early, he is saying that athletic success often makes itself apparent very early in a person's life, and someone who achieves great victories in this field may be hailed as a hero when still very young. In the next line, however, he compares the laurel to the rose, a flower which is often described as short-lived and is associated with the bloom of youth. If we assume that the rose here represents the athlete's youth and good health, then, Housman is saying that the laurel—representative of athletic success and excellence—"withers" even more quickly than youth does in general, which means that it withers very quickly indeed. Even before an athlete's youth is over, his athletic success will have begun to wane, so while the laurel may grow more quickly and make itself known very soon, it will also die very quickly.