elderly vs senior

elderly and senior both are nouns.

Word	Noun	Adjective	Verb	Adverb
elderly	Yes	Yes	No	No
senior	Yes	Yes	No	No

As adjectives, elderly and senior are synonyms defined as:

elderly and senior: advanced in years; (`aged' is pronounced as two syllables)

Other synonyms of elderly include aged, older.

elderly (noun)	senior (noun)
people who are old collectively	an undergraduate student during the year preceding graduation
	a person who is older than you are

elderly (adjective)	senior (adjective)
advanced in years; (`aged' is pronounced as two syllables)	older; higher in rank; longer in length of tenure or service
	used of the fourth and final year in United States high school or college
	advanced in years; (`aged' is pronounced as two syllables)