If you're 18 or elder you're considered mature enough to pay taxes and even vote. But there’s no car rental company offering car rental under 18 Barcelona, Spain.
Sorry, but this is a common age policy of car rental companies all over the world, and you have to accept it.
Car rental under 18 Barcelona, Spain users are not eligible to sign a contract and if the renter decided not to pay, it would bring a huge risk to the car rental company.
Besides, any car rental company should be responsible if a car rental under 18 Barcelona, Spain driver was damaged in an accident or damaged someone else. So, you won't find a car rental company in the United States or anywhere else which will rent car to a user under 18.