Despite the old population, Italy still has one of the more advanced medical systems in all of Europe. Furthermore northern Italy is the wealthiest part.
In terms of respiratory viruses such as COVID-19, there's multiple factors that put people at risk aside from age. One is obesity, and well, if you take a look around this country, we've got plenty of that to go around, which doesn't boast well for rapidly spreading cases. I'm still rather confused at this comparison to the flu. The concern is with how fast COVID-19 spreads from person to person. Each carrier of the flu spreads it to on average 1.3 people, meanwhile each carrier of COVID-19 spreads it to 2.5 people on average. Not to mention the higher fatality rate. Italy having 300+ deaths per day would be much worse than any flu day even in the United States. Furthermore there methods of flu treatment are much more effective, and even if not entirely effective for the majority of the population, we have a vaccination for the flu. You can't even compare the two whatsoever