senior vs elderly

senior and elderly both are nouns.

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

As adjectives, senior and elderly are synonyms defined as:

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

Other synonyms of senior include aged, older.

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

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