The answer to that question depends on the nature and severity of your child's disability. Each child is different, so each child's situation will require an independent evaluation. As the parent, you are the natural guardian of your child until he or she reaches the age of eighteen. As the natural guardian, you have the right to make medical, housing, and other decisions for your child. But when your child turns eighteen, thus reaching the age of majority, those rights are transferred to your child.
If you need to retain decision-making authority, you will have to get it from a court order. A child with a disability severe enough to cause total incapacity, will need a full guardianship, under which all decision-making rights will be removed from the child and given to you. You will have to hire a guardianship attorney who can guide you through the process in your local probate court. Once you are appointed guardian, you will once again have full authority over your child and will be able to make all decisions on his or her behalf.