This question came up about a year ago, although it's a good question that can certainly merit more discussion.

For Italy, the short answer is, "When Napoleon crowned himself King of Italy." Prior to then the concept of an Italian language and culture was solely the purview of a small intellectual elite, who would have nonetheless identified more with their local identity more so than a common one spanning the whole peninsula.

This is actually a very popular issue in Italy; a cohesive 'Italian Identity' has yet to be fully established, and while many people may call themselves Italians out of choice, many people are also more tempted to refer to themselves in terms of their respective municipalities or cities. Sicilian, Neapolitan, etc. There's a large divide between southern Italy and northern Italy as well. Some areas of italy don't even speak the same dialect; there's Lombard, Tuscan, Neopolitan, Sicillian, and waaay more minority languages in Italy all dependent on region.

Italian unification into a single republic is an incredibly recent thing, having happened in the 1940s. Before this, Italy was divided into city states which each had their own ruler and were all more or less independent of one another as governing bodies, and even more before that there were the papal states. Italian cities all developed very uniquely due to separate ruling, so there is not really a collective 'Italian culture' either. The papal states, for instance, were much different than the other states; their governments for a long time were led by the Pope and struggled against the communal form of government city states had, and eventually began to battle with Lombardy and many others for authority. Venice had trade with the east that made her incredibly wealthy, which allowed it to proliferate and especially thrive off of the renaissance that Florence began. Italy's lack of unification was even an attractor for conquerors like Charles VIII who kickstarted the Italian Wars. The wide variety of conflict in Italy meant that cultural boundaries became pretty sharp.

The first real hope for unification in Italy came when Napoleon took the Italian Peninsula and instated the same legislation and currency for the whole of Italy, something that had not happened since the original Roman empire. After Napoleon's defeat, the Risorgimiento movement began, which was the gradual settling of Italy into a single kingdom. Despite papal resistance, the pope agreed to make Rome the capital in 1929, fundamentally uniting every major city in the peninsula. After WW2, a new constitution was made to get rid of the fascism, which was the construction of the current republic.

So I guess the short answer is The Napoleonic Era, and the long answer is We're Still Working On It,