Is a person with mental disability obliged to perform ghusl?
The condition for being accountable and responsible in religious terms is to be sane and to have reached puberty. Everyone with these two qualifications is considered as religiously responsible.
As people with mental disabilities do not have this qualification, they are not responsible for religious duties such as ghusl, ablution, prayer, etc. (Tirmidhi, Hudud, 1; Bukhari, Kashf al-Asrar, IV, 371-373).
Source: Presidency Of Religious Affairs The Turkey, High Board of Religious Affairs FATWAS