graybeard vs old man

graybeard and old man both are nouns.

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

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

Other synonyms of graybeard include greybeard, Methuselah.

graybeard (noun)	old man (noun)
a man who is very old	an informal term for your father
	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