Than is both a subordinating conjunction, as in She is wiser than I am, and a proposition, as in She is wiser than me. As a subject of the clause introduced by the conjunction then, the pronoun must be nominative, and as object of the preposition than, the following pronoun must be in the objective case. Since the following verb am is often dropped or 'understoood,' we regularly hear than I and than me. Some commentators believe that the conjunction is currently more frequent than the preposition, but both are unquestionably Standard
-Columbia Guide to Standard American English, from here

