Actually I don't think paying for the plan is the problem. There are many prepaid providers in the US. Many retail stores sell inexpensive phones and prepaid

Actually I don't think paying for the plan is the problem. There are many prepaid providers in the US. Many retail stores sell inexpensive phones and prepaid service for cash (hell even my local grocery store sells them). I tried several of these recently with my new Nexus 5. T-Mobile didn't even ask for anything other than a first name when activating and I got the impression that was so the customer service rep could call me by name. So if obtaining service is really not the problem what is? Quite simply the problem is that most of these prepaid services don't sell the phones that a teenager is going to want (smart phones in general but iPhones in particular). It's only very recently that these sorts of phones have been supported at all by the prepaid industry. Even when the phones are supported the American phone market is distorted by carrier subsidies. Many people don't realize how much cheaper buying a phone and using a cheaper service can be. The prepaid market on the other hand is not significantly subsidized (if at all). So kids and/or parents don't even consider buying a phone outside of a postpaid provider on a contract. It's possible to buy a used phone for a fairly reasonable price, but then you're dealing with a potential mess of issues. You have to make sure to buy a phone that your chosen carrier will activate. Even some seemingly compatible systems won't activate phones sold by the other (Verizon & Sprint in particular). The phone might work and be allowed by the provider, but not have the proper bands for 3G/4G service (some iPhones on T-Mobile). Your carrier might refuse to activate service since they might say the phone is still under contract or stolen. For that matter the phone might still be carrier locked to another provider. Even if a teenager has the cash to pay for their plan they often won't have the cash to buy the phone they want. So I suspect a lot of parents buy phones. As others have pointed out a lot of the carriers have seemingly low charges for adding additional lines. So combined with the additional line charge and the subsidized cost parents just add their kids to their own postpaid service. Often times though these additional line fees are misleading, since they typically tack on more fees for data capabilities. Which racks up the price. Since this isn't clearly advertised a lot of people don't understand it and don't consider it in price comparisons with prepaid providers. So in my opinion it's almost entirely about phone selection and phone subsidization. Those things drive tons of people to postpaid plans which of course do have the issues you point out. But as soon as you are willing to accept less fashionable phones those sorts of issues erode and there's nothing stopping a teenager from obtaining service (or for that matter even paying for a phone).