kindergartener vs youngster

kindergartener and youngster both are nouns.

Word	Noun	Adjective	Verb	Adverb
kindergartener	Yes	No	No	No
youngster	Yes	No	No	No

As nouns, youngster is a hypernym of kindergartener; that is, youngster is a word with a broader meaning than kindergartener:

Other hypernyms of kindergartener include child, fry, kid, minor, nestling, nipper, shaver, small fry, tiddler, tike, tyke.

kindergartener (noun)	youngster (noun)
a child who attends a preschool or kindergarten	a young person of either sex