When the premium is not mandated by the government, it is usually derived from the calculations of an actuary, based on statistical data. The premium can vary depending on many factors that are believed to affect the expected cost of future claims. Those factors can include the car characteristics, the coverage selected (deductible, limit, covered perils), the profile of the driver (age, gender, driving history) and the usage of the car (commute to work or not, predicted annual distance driven).
If the younger driver is covering his own costs, it might be cheaper for him to take out his own policy but include one of the older, lower risk drivers from the household as a named driver (as long as the older drive will use the car). However, if the older driver is footing the bill for both policies, multi-car insurance may still work out cheaper.
Teenage drivers who have no driving record will have higher car insurance premiums. However, young drivers are often offered discounts if they undertake further driver training on recognized courses, such as the Pass Plus scheme in the UK. In the US many insurers offer a good-grade discount to students with a good academic record and resident-student discounts to those who live away from home. Generally insurance premiums tend to become lower at the age of 25. Some insurance companies offer "stand alone" car insurance policies specifically for teenagers with lower premiums. By placing restrictions on teenagers' driving (forbidding driving after dark, or giving rides to other teens, for example), these companies effectively reduce their risk.
The law 990/1969 requires that each motor vehicle or trailer standing or moving on a public road have third party insurance (called RCA, Responsabilità civile per gli autoveicoli). Historically, a part of the certificate of insurance must be displayed on the windscreen of the vehicle. This latter requirement was revoked in 2015, when a national database of insured vehicles was built by the Insurance Company Association (ANIA, Associazione Nazionale Imprese Assicuratrici) and the National Transportation Authority (Motorizzazione Civile) to verify (by private citizens and public authorities) if a vehicle is insured. There is no exemption policy to this law disposition.
Multi car insurance is designed for households with more than one car, and you can usually cover up to a maximum of five cars on one policy. You can think of it as a family car insurance policy, where each additional vehicle on the policy earns a further multi car discount from the insurer. While some insurers will only cover cars registered to the same address, the best multi car insurance policies will also allow you to cover cars driven by your immediate family, even if they live elsewhere.
The minimum coverage defined by German law for car liability insurance / third party personal insurance is €7,500,000 for bodily injury (damage to people), €500,000 euro for property damage and €50,000 for financial/fortune loss which is in no direct or indirect coherence with bodily injury or property damage. Insurance companies usually offer all-in/combined single limit insurances of €50,000,000 or €100,000,000 (about €141,000,000) for bodily injury, property damage and other financial/fortune loss (usually with a bodily injury coverage limitation of €8-15,000,000 for each bodily injured person).
But liability coverage levels come in threes — you’ll probably see something like 50/100/50 up to 250/500/250 in typical policies. You can think of these limits like: individual injuries / total injuries / property damage. Insurers are a little more technical, calling them bodily injury liability, total bodily injury liability and physical damage liability.