I went supermarket shopping today and had the rudest teenage boy as a cashier......
You were absolutely right to go to the manager. And I don't blame you for walking away from that disaster of a cashier.
At our store (I'm pretty sure by state law) cashiers must be at least seventeen years old. Baggers can be hired as young as fourteen, but they are not working the registers. They have to be at least fifteen to wrangle carts.
I consider myself lucky to have a good working relationship with our two baggers, Kenny and TJ. I treat them as adults, not as kids, and they appreciate that kind of respect. We have a new cashier, age 17, named Caitlyn, who is similarly responding to the adult treatment. And dang, she's got the smarts!
The young workers who don't show any respect for the customers get weeded out fairly quickly. Your cashier wouldn't last a week at our store.