Many kids don't even have a DL when they're 18, let alone a car. How in the hell would a typical 18 year old kid afford a car in Poland without well to do parents? Somehow I doubt working 20 hours a week at Zabka is enough to pay for insurance, gas, repairs, car payments......
Doesn't mean they don't go for the driving license as soon as. I know people who got driving licences in 1991 or so and haven't driven since - still got the quali though.
As for now, our nearest Tesco is on the same road as the driving examination centre. Judging by the people with L plates I see driving to/from there, that driving test is still a young person's rite of passage, car or no car at the end of the deal.
Remember: in PL a car is insured to be driven by anyone; it's not like eg GB where your car insurance is tied to you and 2006 Ford Galaxy, plates FG 22223 or whatever. So you can drive your mate's/parents'/aunt's old banger whenever.