It’s generally required for a parent to co-sign a minor’s bank account in the US. Once you turn 18, the best course of action is to establish an account in your name ONLY so you have sole control of it. It would even be better if you can establish the account at a different bank/credit union than the one the minor account was in, to avoid any inadvertent connections between the previous and new account. Bank personnel can make mistakes and link up what they thought were similar accounts, or give close relatives unwelcome access to accounts in an attempt to be “helpful”.
There are several reasons for this. Stories of people who are still using the accounts they had when they were minors who are shocked when their money is suddenly taken away for reasons beyond their control are not uncommon. Parents can have financial problems and either use the money to pay off their debts or the money is seized by the institutions that they owe. There could be disagreements between parents and their kids over the kid’s life choices, so they take the money away as a punishment or for control. It could just be selfishness and greed, so the parents decide to just take the money. Who earned the money in the account doesn’t matter. If two people are named on the account, the money belongs to both parties and the bank isn’t going to stop someone on the account from withdrawing the cash. Getting it back can be a difficult legal process, if it’s even possible.
Having your own account does not mean that parents can’t send money if their child needs it. All they need is the account and routing number (the same information that would be on a check) to deposit money into the account. In addition, PayPal, Zelle, Venmo, or any number of banking apps are available they could use to send money if they’re still financially supporting their kids or even just want to occasionally send some money as a gift. Other excuses may have good intentions at heart, but from a safety and security standpoint, it’s best to establish an independent banking account.