old boy vs old person

old boy and old person both are nouns.

Word	Noun	Adjective	Verb	Adverb
old boy	Yes	No	No	No
old person	Yes	No	No	No

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

Other hypernyms of old boy include golden ager, oldster, senior citizen.

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