age group vs young

age group and young both are nouns.

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

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

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

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

age group (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