Once your child becomes 18, he is a legal adult, and he should have an estate plan including a health proxy, power of attorney, and a Will. Because he has now reached maturity, HIPPA prevents you, as his parent, from even getting medical updates in the event that your child is unable to communicate his wishes to have you involved.
You may remember some high-profile national cases regarding the right to die of younger individuals. The names: Nancy Cruzan, Karen Quinlan, and Terri Schiavo are examples of such individuals who did not have the proper documents that would have prevented years of litigation regarding their care. All of that could have been avoided for the families if the young adults had signed a written statement of their wishes.
Also, you may have a child with a disability that prevents her from living a normal life, but she may be able to choose how she wants her assets to be distributed. She may choose a program that attended to her needs over years, or perhaps a favorite charity to receive at least a portion of her estate upon her demise. She may also wish to leave funds to her siblings instead of you, her parent.
Your special needs child's capacity to make decisions relative to her own health care and financial future may be determined by her treating physician, a psychologist, or a psychiatrist. She need not be 100% competent, but rather, she needs to understand the nature of what her decisions mean and who she wants to attend to her decisions if she becomes incapacitated.
Sometimes your disabled child is deemed to be incompetent, and a guardian and conservator may be appointed, which basically removes a decision-making power from your child. However, a situation may exist where your child does not have total capacity to make all decisions for himself, but he is sufficiently competent to understand the nature of what the decisions mean, and she may wish to sign a health proxy and power of attorney, which should eliminate the need for the guardianship or conservatorship, thus maintaining privacy, eliminating the probate court system, and alleviating the need for accounts and bonds in the court, etc.
You, as parent, should discuss with your child whether or not she wishes to be kept alive by heroic means if she would not have a meaningful quality of life. Other issues such as organ donations, burial instructions, cremation, etc. may also be determined by your child, as well as other important decisions as to who may receive their important tangible property and financial assets.
It is important to note that not every person has the capacity to make decisions, but in a situation where your child has the ability to state her preferences, goals, and objectives, and also supply input as to whom would make her decisions, her input should be considered. Then it's important to prepare and sign the documents to avoid the probate process.
You should also be aware that often a parent or grandparent has given funds to a minor, and upon the age of 18, these funds are vested and owned by the young person. In the unfortunate event that your child should predecease you, these assets may have to be probated and will pass to that person's heirs-at-law, normally you, his parent. In many situations, you have divested assets in order to reduce your estate for estate tax purposes or for asset protection purposes, and the receipt of assets merely frustrates the initial plan. To accomplish the initial goal it is important for your child to establish an estate plan, which may be as simple as a straightforward will, directing that the assets be left to individuals other than you, possibly siblings, or a charity.