Youth is the time of life between childhood
Childhood is the age span ranging from birth to adolescence. In developmental psychology, childhood is divided up into the developmental stages of toddlerhood , early childhood , middle childhood , and adolescence .- Age ranges of childhood :The term childhood is non-specific and can imply a...

 and adult
An adult is a human being or living organism that is of relatively mature age, typically associated with sexual maturity and the attainment of reproductive age....

hood (maturity
Maturity (psychological)
Maturity is a psychological term used to indicate how a person responds to the circumstances or environment in an appropriate manner. This response is generally learned rather than instinctive, and is not determined by one's age...

). Definitions of the specific age range that constitutes youth vary. An individual's actual maturity may not correspond to their chronological age, as immature individuals could exist at all ages.
Around the world, the terms "youth", "adolescent", "teenager", "kid", and "young person" are interchanged, often meaning the same thing, occasionally differentiated.

It is an illusion that youth is happy, an illusion of those who have lost it; but the young know they are wretched, for they are full of the truthless ideals which have been instilled into them, and each time they come in contact with the real they are bruised and wounded.

W. Somerset Maugham, Of Human Bondage (1915)

Youth are diamonds in the sun, diamonds are for ever.

Alphaville (band)|Alphaville, Forever Young (Alphaville song)|Forever Young (1984).

YOUTH: The too-brief span wherein the human chassis is factory-fresh, undented and free of corrosion; a pristine condition worshiped by menopausal women in sweatsuits and shrinking men with chestnut-brown toupees, while those who actually possess it are frequently too shallow or despondent to enjoy it.

Rick Bayan|Rick Bayan, The Cynic's Dictionary, 1994.

Youth cannot know how age thinks and feels. But old men are guilty if they forget what it was to be young... and I seem to have forgotten lately.

Dumbledore in Harry Potter|Harry Potter and the Order of The Phoenix by J.K. Rowling

Age is foolish and forgetful when it underestimates youth.

Dumbledore in Harry Potter|Harry Potter and the Half-Blood Prince by J.K. Rowling

The greatest part of mankind employ their first years to make their last miserable.

Jean de La Bruyère, p. 623.

Use thy youth so that thou mayest have comfort to remember it when it hath forsaken thee, and not sigh and grieve at the account thereof. Use it as the spring-time which soon departeth, and wherein thou oughtest to plant and sow all provisions for a long and happy life.

Sir Walter Raleigh, p. 623.