Your adult children can join or remain on your plan under the law, regardless of whether they are married, living with you, in school, or financially reliant on you. While your adult kid must have coverage, the health plan is not obliged to include your child's spouse or children. The only time it would be illegal for an insurer to provide coverage for your adult child is if that person has been determined to be "uninsurable." If your child becomes ill and cannot work, their health coverage will help them pay for their medical bills. If they do not have coverage, they could be forced to choose between buying health insurance or paying for their own care. This is called "coverage gap" and it's why many people think it's important for all adults, including young adults, to have some form of health insurance. The reason your child can't be denied coverage because of a preexisting condition is called "guaranteed issue". If your child wants to stay on their parent's policy, they just need to show ID that proves they are alive. Even if someone denies them coverage initially, if they later apply and are accepted, there is no way to deny them coverage again once they are insured. Children can also stay on their parents' policy if they move out but still want to be covered by health insurance.