Why the account has to be joint, in the regulators' own words. The five federal banking agencies addressed this directly in their 2015 youth savings guidance. Answering whether there are restrictions on minors opening savings accounts, the agencies wrote: "No federal law prohibits minors from opening savings accounts. Rather, a deposit account relationship is based on a contract governed by state law. In general, minors are deemed to not have the legal capacity to enter into a contract, including opening an account at a financial institution, meaning that a contract with a minor is potentially 'voidable.' However, some states specifically allow a minor to open a savings account. For example, the State of Washington permits a minor to enter into a valid and enforceable contract for a deposit account with a financial institution. States also have different legal definitions of 'minor.'" The bank's exposure is that a voidable contract can be disaffirmed by the minor, so an adult co-owner is the ordinary way institutions get an enforceable agreement. Note the shape of that answer: the barrier is not federal, the rule differs by state, and even the age at which someone stops being a minor is not uniform.
The debit card is what decides the titling, and this is the point readers most often have backwards. Asked whether a minor with a custodial account can be issued an ATM or debit card, the same guidance answers: "The Uniform Transfers to Minors Act or Uniform Gifts to Minors Act of each state governs custodial accounts for minors. As a general matter for custodial accounts, a custodian manages the funds in the account on behalf of a minor, meaning the minor would not be able to withdraw funds without the custodian's approval. Therefore, a minor with a custodial account should not be provided with an ATM or debit card that permits withdrawals." A teen account exists so that the teenager can spend from it. That requirement is incompatible with custodial titling, so the account is titled jointly instead, and the choice between the two structures is driven by the card rather than by any tax or ownership preference.
What joint titling means for whose money it is. In a joint deposit account each co-owner has withdrawal rights, so a parent can move money out as well as in, and depositing money into the account is not by itself a completed gift to the child. That is the practical difference from a transfers-to-minors account, where the transfer is irrevocable and the property is the child's from the start, to be handed over outright at an age the state's statute sets. Families who want the money to be unambiguously the child's are choosing the wrong product with a teen checking account, and families who want the money spendable by the teenager are choosing the wrong product with a custodial account.
Every consumer protection applies, and the guidance lists them. Asked whether consumer protection laws reach accounts held by or for the benefit of minors, the agencies answered that they do, naming the Children's Online Privacy Protection Act, the Electronic Fund Transfer Act and Regulation E, the Expedited Funds Availability Act and Regulation CC, the Truth in Savings Act and Regulation DD, and the prohibitions on unfair or deceptive acts or practices. In practice the ones a teenager meets are Regulation E, which governs liability for an unauthorized debit card transaction and the procedure for reporting one, and Regulation DD, which governs how the account terms and fees must be disclosed.
What happens at majority is a product rule, not a legal event. Nothing in law converts the account when the child turns 18. What happens instead is set by the product: a bank that markets a youth account states an age at which it ends, and at that point the account is converted to one of the bank's standard checking products, on that product's fee schedule, with the co-owner removed if asked. Because the terms are the bank's own, the age and the resulting fees differ between institutions, and the conversion is the moment to compare the standard product against alternatives rather than to accept it by default. The account's history stays with it, which is one reason families open one early: the teenager arrives at adulthood with an existing banking relationship rather than a cold application.