Fountain of Youth vs outflow

Fountain of Youth and outflow both are nouns.

Word	Noun	Adjective	Verb	Adverb
Fountain of Youth	Yes	No	No	No
outflow	Yes	No	No	No

As nouns, outflow is a hypernym of Fountain of Youth; that is, outflow is a word with a broader meaning than Fountain of Youth:

Fountain of Youth: a fountain described in folk tales as able to make people young again
outflow: a natural flow of ground water

Other hypernyms of Fountain of Youth include fountain, natural spring, outpouring, spring.

Fountain of Youth (noun)	outflow (noun)
a fountain described in folk tales as able to make people young again	the discharge of a fluid from some container
	a natural flow of ground water
	the process of flowing out