All the Italian companies I've checked requires the driver to be at least 21 years old and a driving license held for at least an year, claiming that the second one is a European requisite. Problem is, I can't find the exact law anywhere.
But given my long personal experience with renting car, I can tell you that if all the major companies are doing it then they all must have a good reason to do it.
I made some more research, and I cannot find anywhere the normative that rental companies are referring to; mostly because the just say (when they say it) that it's a European thing without any reference at all.
But I've found a possible explanation: there are many limits in place for drivers younger than 21 years old and/or with a driver license held from less than a year, at least in Italy (I found the law reference in their Driving Code). Basically if you fall into those two categories you need a special (more expensive) insurance and can drive only de-powered cars (plus other small tibidids).
That would make really complicate for rental companies to rent cars, because they would have to keep some car only for young and/or new drivers, with a more expensive insurance and a different engine. Now, I don't know if this applies to the entire Europe or not, and in which way...