kindergartner vs youngster

kindergartner and youngster both are nouns.

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

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

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

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