Facebook has no obligation to police content to comply with the laws of any nation except the USA.
Everyone else can fuck right on off. Let the government of Italy (try to) block Facebook if they're actually serious about it.
Leaving aside the international question, even in the US contracts entered into by a minor are considered invalid. Facebook does nothing to try to actively restrict access to legal adults even according to US law. There is nothing legally that Facebook can do to enforce an invalid contract such as their AUP. That is the angle the Italians are taking and to me it seems a valid one.