old man vs old boy

old man and old boy both are nouns.

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

As nouns, old man and old boy are synonyms defined as:

old man (noun)	old boy (noun)
an informal term for your father	a vivacious elderly man
a man who is very old	a former male pupil of a school
a familiar term of address for a man	a familiar term of address for a man
(slang) boss
aromatic herb of temperate Eurasia and North Africa having a bitter taste used in making the liqueur absinthe