In Indiana, whole bunches of laws happen, your parents could not just "say" that and have anything happen, they have to have a lot of backup support, etc.

http://www.ai.org/legislative/ic/cod.../ar26/ch7.html

Your parents can take care of you as long as they want :-) You don't need a law to allow that. If they want special programs for you they should talk to their lawyer or therapist, etc. They'll know. It might get more help for you but it would limit your freedom (lots of rules and regulations) and might or might not be under your parents "control" and they'd have to have interviews with social workers constantly, etc. and people making sure they were good caretakers, etc. It would be the "State" in charge and that's not always good if it can be avoided.