And round that early-laurelled head Will flock to gaze the strengthless dead, And find unwithered on its curls The garland briefer than a girl's.
Now that the athlete has entered the land of the dead, the residents of this "stiller town" (remember line 8?) will "flock" around him to look at his laurel victory crown. It's kind of like that day you showed up at school with those lightning bolts shaved in your head—sweet.
Since the athlete dies at the height of his prowess and fame, the laurel on his head is still nice and fresh.
The poem's last line brings to mind the flower garlands that children (here "girls") make. They last for only a very short, brief, time.
The laurel, the thrill and acclaim of victory, is, in life, a very fleeting thing—lasting for less time than a child's flower garland. But for this athlete, in death, the laurel will remain "unwithered" for eternity.
Neat trick, eh? It's just too bad you have to be dead for it to work.