12th vs twelfth

12th is not a noun while twelfth is a noun.

Word	Noun	Adjective	Verb	Adverb
12th	No	Yes	No	No
twelfth	Yes	Yes	No	No

As adjectives, 12th and twelfth are synonyms defined as:

12th and twelfth: coming next after the eleventh and just before the thirteenth in position

12th (noun)	twelfth (noun)
	one part in twelve equal parts
	position 12 in a countable series of things

12th (adjective)	twelfth (adjective)
coming next after the eleventh and just before the thirteenth in position	coming next after the eleventh and just before the thirteenth in position