old boy vs senior citizen

old boy and senior citizen both are nouns.

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

As nouns, senior citizen is a hypernym of old boy; that is, senior citizen is a word with a broader meaning than old boy:

Other hypernyms of old boy include golden ager, old person, oldster.

old boy (noun)	senior citizen (noun)
a vivacious elderly man	an elderly person
a former male pupil of a school
a familiar term of address for a man