This is something I've noticed in my years of working in the service industry. While teenagers aren't the best behaved of the customers, they are not the worst of them. In fact, far from it. Many of them are actually polite and understanding, or at least straight-forward, no nonsense, types of customers who just want to get in and get out. The worst customers, are the old people. They don't listen when you try to explain things, they argue over your own policies even though you've been trained more recently then they have, they try to tell you how to do your job and don't have any faith in your skills no matter how fucking awesome you are at it, and they never remember how to run their checks even though they've been using this stupid, asinine, and outdated method of payment since before I was born, and often times expect things out of you that aren't part of your job requirement. I mean, honestly, they will wait five minutes fiddling with their wallet and purse much longer then any normal person does and wait for you to put their bags in their cart. I wanna scream "that's not part of my cashier's jobs. They're not doing it to be nice, they're doing it because you take too fucking long." and the worst part is, I'm pretty sure some of them know they're supposed to do it themselves but don't because they know that the cashiers will if they wait long enough because they think they're entitled to it or something. Good service isn't a right, it's a nicety that we provide to your wrinkly self-centered asses. If it were up to me, you'd all be using the self-checkouts.
And that's another thing, why does everyone have to bitch about them. Not just their performance, I could live with that. But they bitch about their very existence, About how they're taking people's jobs away and other such misinformed bullcrap. Did people bitch this much about self-serve gas pumps? Seriously. They still need a cashier to attend to them to deal with problems, and at my store, I wish we had more because we don't have enough cashiers to keep up with the demand all the time because apparently getting people to actually show up for the job is surprisingly difficult.
Besides, they'll never completely replace normal registers...not before both are replaced by automated storefronts at least, something that will be coming in the next 10-20 years. Basically a super sized vending machine, with conveyer belts that move food items from storage to a deposit where a person can pick up whatever they ordered. Chips, milk, soda, even alcohol thanks to a camera and a person checking the ID by remote. That's right...ID checks will be outsourced. Convenience and grocery stores will eventually be replaced by them, leaving only clothing and electronics stores...maybe.