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.
Auto insurance is financial protection, and not just for the investment you made when you bought your car. After a really serious accident, bills for damage and injuries can easily reach into hundreds of thousands of dollars. If you happen to cause such a wreck, the victims could sue you. In the worst case scenario, assets such as your savings and home could be seized.