Definition of "youth"

Serene, smiling, enigmatic, she faced him with no fear whatever showing in her dark eyes. The clear light of the bright autumn morning had no terrors for youth and health like hers.

1910, Emerson Hough, “The Purchase Price”, in The Purchase Price: Or The Cause of Compromise, Indianapolis, Ind.: The Bobbs-Merrill Company

Feel awfully about Scott... It was a terrible thing for him to love youth so much that he jumped straight from youth to senility without going through manhood. The minute he felt youth going he was frightened again and thought there was nothing between youth and age.

1936 Feb. 15, Ernest Hemingway, letter to Maxwell Perkins

I don't find the pose of careless youth charming and engaging any more than you find the pose of careworn age fascinating and eccentric, I should imagine.

1991, Stephen Fry, The Liar, London: Heinemann, page 49

Toward the end of the war, Benoit was sent off on his own with forged papers; he wound up working as a horse groom at a chalet in the Loire valley. Mandelbrot describes this harrowing youth with great sangfroid.

2013 January, Brian Hayes, “Father of Fractals”, in American Scientist, volume 101, number 1, page 62

[…] and then a youth appeared—no one quite knew where from or to whom he belonged—but he settled down with them in a happy-go-lucky way, and they all lived together.

1919, W[illiam] Somerset Maugham, chapter LII, in The Moon and Sixpence, [New York, N.Y.]: Grosset & Dunlap Publishers […], pages 274–275