The elves in The Two Towers may look eternally young, but the characters who really symbolize youth—the innocence, the cheerfulness, the lack of experience—are clearly the hobbits in general and Merry and Pippin in particular. Their ability to take pleasure in ordinary things, their upbeat attitude, and their general exuberance provide a pleasant contrast to the dark goings on about Middle-earth. And not just any of the hobbits, since Frodo is getting older and older-seeming as the Ring starts to bother him more. Without their cheer, this book just might turn into Mordor-Mordor-doom-doom-doom on repeat. But while the presence of the hobbits in The Lord of the Rings keeps it from getting too epic and too serious, they serve a greater thematic purpose, too. In the face of great evil, the hobbits manage to maintain their sense of joy and good, and that's what makes them heroes.
Pippin and Merry's youth gives them access to serious characters like Treebeard and Théoden, who might be more suspicious of unknown adults.
In The Two Towers, Gollum's recollections of the folk tales and customs of his younger days prove that Sméagol—Gollum's good side—is rising to the surface under Frodo's care. Too bad Sam had to go muck it up.