In English Language and Usage there is an interesting question about immature adults.
Some of the terms that might fit in Italian, with different nuances of course, are "bamboccione" from the famous neologism introduced by Tommaso Padoa Schioppa, "vitellone" (from F. Fellini's movie) and "mammone" (mama's big boy).
It seems that these terms are synonyms or at least hyponyms of "immature (adult)"; they have been used more frequently in different periods of time, but essentially with the similar, very close meaning of immature adult person (usually a man) who will not grow up, a bit of a slacker, lacking any ambition or autonomy, even "un fannullone" and/or "un buono a nulla".
Are there other synonyms/hyponyms of "immature (adult)" that have been famous for a little while and then discontinued/replaced?