There is a little known Social Security benefit called the Adult Child Disability benefit. I took on one such claim this week, for the first time in quite a while.
An adult disabled before age 22 may be eligible for child's benefits if a parent is deceased or receives retirement or disability benefits. The SSA considers this a "child's" benefit because it is paid on a parent's Social Security earnings record. This benefit is usually more favorable for the claimant than SSI payments.
The "adult child" must be unmarried, age 18 or older, and have a disability that started before age 22.
The SSA makes the disability determination using the usual adult disabilty process.