age class vs stratum

age class and stratum both are nouns.

Word	Noun	Adjective	Verb	Adverb
age class	Yes	No	No	No
stratum	Yes	No	No	No

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

Other hypernyms of age class include class, social class, socio-economic class.

age class (noun)	stratum (noun)
people in the same age range	an abstract place usually conceived as having depth
	a subpopulation divided into a stratified sampling
	people having the same social, economic, or educational status
	one of several parallel layers of material arranged one on top of another (such as a layer of tissue or cells in an organism or a layer of sedimentary rock)