not bad vs old

not bad is not a noun while old is a noun.

Word	Noun	Adjective	Verb	Adverb
not bad	No	Yes	No	No
old	Yes	Yes	No	No

As adjectives, not bad and old are synonyms defined as:

Other synonyms of not bad include bang-up, bully, corking, cracking, dandy, great, groovy, keen, neat, nifty, peachy, slap-up, smashing, swell.

not bad (noun)	old (noun)
	past times

not bad (adjective)	old (adjective)
excellent	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