old vs gray-haired

old is a noun but gray-haired is not a noun.

Word	Noun	Adjective	Verb	Adverb
old	Yes	Yes	No	No
gray-haired	No	Yes	No	No

old (noun)	gray-haired (noun)
past times

old (adjective)	gray-haired (adjective)
of long duration; not new	showing characteristics of age, especially having grey or white hair
(used especially of persons) having lived for a relatively long time or attained a specific age
skilled through long experience
(used for emphasis) very familiar
excellent
just preceding something else in time or order
belonging to some prior time