As a children reach the legal age of adulthood, the law presumes them competent to take full responsibility for their own affairs. To extend control beyond this age requires direct action by the person seeking guardianship to prove that the adult child is incapable of functioning independently. Different types of guardianship are available to address the wide range of differing needs and abilities of disabled adults.
Assess the disabled child's skills and abilities. Particularly consider the ability to perform basic daily living tasks and higher order skills such as the abilities to participate in legal contracts and manage finances. Absolute guardianship may not be necessary and may not be in the best interests of the child.
Work with the child to enable him to understand guardianship and why you believe it to be in his best interests. This will reduce the chance that the child will contest your petition and assist you in understanding what issues will need to be addressed if he does.
File a petition in your state court detailing why guardianship is appropriate. It is best to seek the help of an attorney who specializes in the issues facing parents of disabled children.