Claiming Disabled Adults as Dependent

It depends on the facts and circumstances. The dependent exemption was suspended beginning in 2018 with Tax Reform, but taxpayers may qualify for a new Other Dependent Credit (ODC) for certain adult dependents. There are rules to qualify for the ODC which can be found in Pub 501, chart on page 10-11:

https://www.irs.gov/pub/irs-pdf/p501.pdf 

The term “dependent” means a qualifying child, or a qualifying relative. 

1. The person can't be your qualifying child or the qualifying child of any other taxpayer.
2. The person either (a) must be related to you in one of the ways listed under Relatives who don't have to live with you, or (b) must live with you all year as a member of your household (and your relationship must not violate local law). 

3. The person's gross income for the year must be less than $4,150

4. You must provide more than half of the person's total support for the year.

@BabyBlues9 cast your baby blues on the two previous answers. Permissions have nothing to do with it.