greybeard vs old man

greybeard and old man both are nouns.

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

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

Other synonyms of greybeard include graybeard, Methuselah.

greybeard (noun)	old man (noun)
a stoneware drinking jug with a long neck; decorated with a caricature of Cardinal Bellarmine (17th century)	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