senescent vs aging

senescent is not a noun while aging is a noun.

Word	Noun	Adjective	Verb	Adverb
senescent	No	Yes	No	No
aging	Yes	Yes	No	No

As adjectives, senescent and aging are synonyms defined as:

Other synonyms of senescent include ageing.

senescent (noun)	aging (noun)
	the organic process of growing older and showing the effects of increasing age
	acquiring desirable qualities by being left undisturbed for some time

senescent (adjective)	aging (adjective)
growing old	growing old