old man vs Methuselah

old man and Methuselah both are nouns.

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

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

Other synonyms of old man include graybeard, greybeard.

old man (noun)	Methuselah (noun)
an informal term for your father	(Old Testament) a patriarch (grandfather of Noah) who is said to have lived 969 years
a man who is very old	a man who is very old
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