youngness vs youth

youngness and youth both are nouns.

Word	Noun	Adjective	Verb	Adverb
youngness	Yes	No	No	No
youth	Yes	No	No	No

As nouns, youth is a hyponym of youngness; that is, youth is a word with a more specific, narrower meaning than youngness:

Other hyponyms of youngness include juvenility, youthfulness, childishness, puerility.

youngness (noun)	youth (noun)
the opposite of oldness	the freshness and vitality characteristic of a young person
	young people collectively
	a young person (especially a young man or boy)
	early maturity; the state of being young or immature or inexperienced
	the time of life between childhood and maturity
	an early period of development