small fry vs youngster

small fry and youngster both are nouns.

Word	Noun	Adjective	Verb	Adverb
small fry	Yes	No	No	No
youngster	Yes	No	No	No

As nouns, small fry and youngster are synonyms defined as:

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

small fry (noun)	youngster (noun)
someone who is small and insignificant	a young person of either sex
a young person of either sex