age bracket vs youth

age bracket and youth both are nouns.

Word	Noun	Adjective	Verb	Adverb
age bracket	Yes	No	No	No
youth	Yes	No	No	No

As nouns, youth is a hyponym of age bracket; that is, youth is a word with a more specific, narrower meaning than age bracket:

Other hyponyms of age bracket include aged, elderly, young.

age bracket (noun)	youth (noun)
a group of people having approximately the same age	the freshness and vitality characteristic of a young person
	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