While the other answer correctly states the limitations for domestic licenses, international driving licenses are governed by international treaties.
If you hold an International Driving Permit according to the Geneva Convention it should be completely valid for the class of vehicles noted on the IDP.
To be on the safe side, however, you should assume that at least the reduced speed limits for a new driver do apply (as does the code of the road).
International driving licenses are only required for non-EU drivers - if you have an EU license, it is basically treated the same as an Italian one.
However, you need to be at least 18 years old. This is the minimum legal driving age (for cars) in Italy; and the international conventions explicitly allow to refuse younger drivers. You must also not be a resident in Italy - once you take residence, you're required to obtain an Italian license.
The car needs to be "legal" as well (so all the bits about insurance do always apply).
Even if you're legal to drive, car rentals can (and will) refuse to rent or surcharge you if you're young (e.g. under 25) and/or a new driver.
 Note that some nations (like Russia, Germany and Mexico) also issue permits under the Vienna Convention, and cross-acceptance between nations using different conventions can sometimes be murky; though Italy should supposedly accept both.
(Edited to add information about EU licenses and cautions)