youth vs period of time

youth and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
youth	Yes	No	No	No
period of time	Yes	No	No	No

As nouns, period of time is a hypernym of youth; that is, period of time is a word with a broader meaning than youth:

Other hypernyms of youth include period, time period.

youth (noun)	period of time (noun)
the freshness and vitality characteristic of a young person	an amount of time
young people collectively
a young person (especially a young man or boy)
early maturity; the state of being young or immature or inexperienced
the time of life between childhood and maturity
an early period of development