"How beautiful is youth! how bright it gleams with its illusions, aspirations, dreams! Book of Beginnings, Story without End, Each maid a heroine, and each man a friend!" is a wonderful quote by Henry Wadsworth Longfellow.
Another would be Nathaniel Hawthorne's, "Man's own youth is the world's youth; at least he feels as if it were, and imagines that the earth's granite substance is something not yet hardened, and which he can mold into whatever shape he likes."
There are so many, for example Pearl S. Buck's, "The young do not know enough to be prudent, and therefore they attempt the impossible -- and achieve it, generation after generation."
Another is, "There is a period near the beginning of every man's life when he has little to cling to except his unmanageable dream, little to support him except good health, and nowhere to go but all over the place." E. B. White
"Youth, with swift feet, walks onward in the way; the land of joy lies all before his eyes." is yet another good one from Edward Bulwer-Lytton, 1st Baron Lytton.
I hope that among these you may find something you like.