youth vs age group

youth and age group both are nouns.

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

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

Other hypernyms of youth include age bracket, cohort.

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