age group vs people

age group and people both are nouns.

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

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

age group: a group of people having approximately the same age
people: (plural) any group of human beings (men or women or children) collectively

age group (noun)	people (noun)
a group of people having approximately the same age	(plural) any group of human beings (men or women or children) collectively
	the body of citizens of a state or country
	members of a family line
	the common people generally

age group (verb)	people (verb)
	fill with people
	furnish with people