Hello
The IRS defines a dependent as a qualifying child (under age 19 or under 24 if a full-time student, or any age if permanently and totally disabled) or a qualifying relative.
Therefore to answer your question specifically , an adult child with a disability who cannot work would qualify as a dependent if they meet the other dependency requirements.
To be a qualifying child
- must have lived with you for half the year
- must have provided more than half their support
- must be related
- meet the age requirement unless permanently disabled.
To be a qualifying relative:
- must have provided more than half support
- must not make over 4400
- must live with you over half the year ( unless on list of persons who dont have to live with you )
I have included a link with more information on the rules for claiming a dependent on your tax return:
https://turbotax.intuit.com/tax-tips/family/rules-for-claiming-a-dependent-on-your-tax-return/L8LODbx94