I have a 19yr old son that has an autism spectrum disorder. He will stay in a public school program until he turns 21. We are in the process of applying for ssi-disability for him. I was told by a friend that after a child turns 18, you have to apply for legal guardianship for them. I was very surprised at this. Is this true?

Generally an adult guardianship is used when an individual is impaired by mental illness, mental deficiency, physical illness or disability, chronic use of drugs, chronic intoxication or other cause so that they lack sufficient understanding or capacity to make or communicate informed decisions and need someone to make such decisions for them.

Any person interested in the adult’s welfare may petition for the appointment of a guardian. An interested person or the subject of a guardianship petition may file an answer to the petition and either object to the guardianship altogether or ask the court to order different relief than requested in the petition.

It depends on whether he is physically or mentally incapable of making important decisions for himself or not.

When a person turns 18+ in the eyes of the law he/she is an adult./there is no difference between people with or without disabilities/.You must apply for guardianship in order to legally make medical and financial decisions for him.It is an arrangement through which you(the guardian) is legally authorized to make decisions for your son (the ward).
For young adults with severe developmental disabilities, the need to seek guardianship is obvious. However, a young adult with mild delays might only need limited guardianship or conservatorship.