It's called an A-traktor. They are restricted to 30 km/h, rear seat and trunk have to be inaccessible, ie only 2 or 3 can ride in the front seats. By the older rules the speed was restricted by two gearboxes with some of the gears blocked because of that only RWD cars could be used. The car also had to be cut up and rebuilt into a pickup. Now a days they are restricted electronically and you don't have to convert it into a pickup. There was a massive boom after the change of the rules with lots of Volvo V70 and BMWs popping up on the roads. There has been a few accidents with these as the restriction is easily manipulated and driven by inexperienced kids without a proper drivers license.
There has been alot of debate whether 15 year olds really should be allowed to clog up traffic with what is basically a car. There is also talk about that the EU want to ban these tractors.