youthful vs young

youthful is not a noun while young is a noun.

Word	Noun	Adjective	Verb	Adverb
youthful	No	Yes	No	No
young	Yes	Yes	No	No

As adjectives, youthful and young are synonyms defined as:

Other synonyms of youthful include vernal.

youthful (noun)	young (noun)
	any immature animal
	young people collectively

youthful (adjective)	young (adjective)
suggestive of youth; vigorous and fresh	(used of living things especially persons) in an early period of life or development or growth
	(of crops) harvested at an early stage of development; before complete maturity
	not tried or tested by experience
	being in its early stage
	suggestive of youth; vigorous and fresh