sometime vs old

sometime is not a noun while old is a noun.

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

As adjectives, sometime and old are synonyms defined as:

Other synonyms of sometime include erstwhile, former, one-time, onetime, quondam.

sometime (noun)	old (noun)
	past times

sometime (adjective)	old (adjective)
belonging to some prior 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

sometime (adverb)	old (adverb)
at some indefinite or unstated time