Italian vs Old Italian

Italian and Old Italian both are nouns.

Word	Noun	Adjective	Verb	Adverb
Italian	Yes	Yes	No	No
Old Italian	Yes	No	No	No

As nouns, Old Italian is a hyponym of Italian; that is, Old Italian is a word with a more specific, narrower meaning than Italian:

Other hyponyms of Italian include Old Italian language, Sardinian, Tuscan.

Italian (noun)	Old Italian (noun)
the Romance language spoken in Italy	the Italian language up to the middle of the 16th century
a native or inhabitant of Italy

Italian (adjective)	Old Italian (adjective)
of or pertaining to or characteristic of Italy or its people or culture or language