Definitions of youth:

• noun:   the freshness and vitality characteristic of a young person
• noun:   early maturity; the state of being young or immature or inexperienced
• noun:   the time of life between childhood and maturity
• noun:   an early period of development

  Example: "During the youth of the project"

• noun:   young people collectively

  Example: "Youth everywhere rises in revolt"

• noun:   a young person (especially a young man or boy)