old vs sometime

old is a noun but sometime is not a noun.

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

As adjectives, old and sometime are synonyms defined as:

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

old (noun)	sometime (noun)
past times

old (adjective)	sometime (adjective)
of long duration; not new	belonging to some prior time
(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

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