where do the Polish youngsters go
In the cities, they go for kebabs and zapiekanki if anything. In the country - they usually can't afford to eat out much so they go for pizza or go home for some proper food.
Are there any typical Polish snacks you can buy in quick-joints?
Yes, zapiekanki - half a long piece of baguette, covered in cheese + other toppings then grilled and covered with ketchup. Also in Krakow you get lots of obwarzanki sold by ladies from little carts dotted around the city.