The general rule is that parents are not responsible for an adult child’s medical debt, but they are responsible for a minor child’s bills.
Confusion about Child-Parent Responsibility for Healthcare Bills
However, who is responsible for an account if the parent completes the initial patient forms when the child is a minor and the child comes back after his / her 18th birthday?
The parents are liable for any existing balance prior to the age of 18. The adult child must complete their own paperwork at the age of 18, because the prior paperwork would have expired. In fact, offices might want to make sure the contract explicitly states the parents are responsible for the entire prior balance, even after the child turns 18.
Future Charges
As far as future charges, the parent would not be responsible, unless the parent agreed to pay. If the parents will pay, some offices use a simple form that parents sign when they agree to be responsible for an adult child's account. Other offices just have the parents write and sign a short statement that stipulates that.
No matter what, though, when the patient turns 18, the adult child still needs to fill out new paperwork. New paperwork is needed from the adult child because they are now the primary responsible party and parents are secondary even if the parents are paying on the account.
Even if your office neglected to have the 18-year old patient do new paperwork, and if the account goes delinquent, the adult child is still the responsible party and you would send the account to collections under the adult child's name.