Now you will not swell the rout Of lads that wore their honours out, Runners whom renown outran And the name died before the man.
Line 17 might have you scratching your head and running for your dictionary (okay, opening your dictionary app). Never fear.
It's not that your vocabulary skills are lacking (but if they are, there's always Shmoop's SAT vocabulary prep).
The problem is the fact that Housman wrote this poem in the late nineteenth century, so some of the vocabulary is a little dated.
The word that's most likely throwing you off is "rout." You're probably familiar with the word's more common meaning: a severe or humiliating defeat. If your school has a really bad football team, you might actually hear the word a lot.
Housman is using "rout" to mean something else entirely. In this context, "rout" refers to a large group or an unruly crowd. This definition of the word doesn't come up too often these days, so that's why it probably tripped you up. If it didn't, good for you. You get props, and a gold star for the day.
So, now that we have that out of the way, the speaker's message in this stanza becomes pretty clear.
He's telling the athlete (the "you") that, since he's dead, he doesn't have to worry about swelling the rout (making the crowd bigger by adding himself to the ranks). What crowd you ask?
It's a crowd of all those "lads," all those young athletes that fame finally passed by ("renown outran"). Their fame "died" before they did. Our dead athlete doesn't have to worry about that. He punched out before the haters could start hating. In our speaker's eyes, this is yet another benefit of dying young.