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It is important to determine if the person is still enjoying the sensation and not causing harm to him-/herself.
If he/she is causing abrasion to the tissues, offering K-Y jelly or some other water-soluble lubricant may be helpful.
If a person has autism and experiences tactile defensiveness, masturbation may look different.
Based on my experience, I believe this can be taught, regardless of the identified level of disability. This type of information helps these individuals know that they are not "falling apart" and that they can ask questions.
By extension, they can feel that their normal sexual feelings are also bad or wrong.