Methuselah vs old man

Methuselah and old man both are nouns.

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

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

Other synonyms of Methuselah include graybeard, greybeard.

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