yearling vs youngster

yearling and youngster both are nouns.

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

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

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

yearling (noun)	youngster (noun)
an animal in its second year	a young person of either sex
a racehorse considered one year old until the second Jan. 1 following its birth
a young child who is just beginning to walk