longtime vs old

longtime is not a noun while old is a noun.

Word	Noun	Adjective	Verb	Adverb
longtime	No	Yes	No	No
old	Yes	Yes	No	No

longtime (noun)	old (noun)
	past times

longtime (adjective)	old (adjective)
having existed or persisted or continued in a particular role or state for a long time	of long duration; not new
	(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