old vs senior

old and senior both are nouns.

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

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

old (adjective)	senior (adjective)
of long duration; not new	older; higher in rank; longer in length of tenure or service
(used especially of persons) having lived for a relatively long time or attained a specific age	used of the fourth and final year in United States high school or college
skilled through long experience	advanced in years; (`aged' is pronounced as two syllables)
(used for emphasis) very familiar
excellent
just preceding something else in time or order
belonging to some prior time