youthfulness vs youth

youthfulness and youth both are nouns.

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

As nouns, youthfulness and youth are synonyms defined as:

youthfulness and youth: the freshness and vitality characteristic of a young person

Other synonyms of youthfulness include juvenility.

youthfulness (noun)	youth (noun)
the freshness and vitality characteristic of a young person	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