patriarch vs old man

patriarch and old man both are nouns.

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

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

Other hypernyms of patriarch include graybeard, greybeard, Methuselah.

patriarch (noun)	old man (noun)
the male head of family or tribe	an informal term for your father
any of the early biblical characters regarded as fathers of the human race	a man who is very old
title for the heads of the Eastern Orthodox Churches (in Istanbul and Alexandria and Moscow and Jerusalem)	a familiar term of address for a man
a man who is older and higher in rank than yourself	(slang) boss
	aromatic herb of temperate Eurasia and North Africa having a bitter taste used in making the liqueur absinthe