age bracket vs young

age bracket and young both are nouns.

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

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

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

age bracket (noun)	young (noun)
a group of people having approximately the same age	any immature animal
	young people collectively

age bracket (adjective)	young (adjective)
	(used of living things especially persons) in an early period of life or development or growth
	(of crops) harvested at an early stage of development; before complete maturity
	not tried or tested by experience
	being in its early stage
	suggestive of youth; vigorous and fresh