There are two sets of rules.
For a "qualifying child" dependent (that's the name of the rule, even if they are not a child), the person must be permanently disabled, live in your home more than half the year, and not provide more than half their own support.
- For the disabled test, disabled means unable to perform gainful work, due to a condition that is permanent, or will last at least one year, or will lead to death. Gainful work is employment for money. Someone who can work a part-time job at the usual wage is generally considered to be able to perform gainful work, even if they need accommodations. And many people with medical disabilities are still able to work, so they don't qualify. For the dependent test, disabled means unable to work.
- For the support test, you don't have to provide more than half, just the person provides less than half. Disability payments in the person's name don't count as support they provide themselves, but earned income, or money they spend from their savings or investments, does count as support they provide to themself. They need to provide less than half their total support.
The other type of dependent is "qualifying relative." For qualifying relative dependent who is your child, you only have to show that you pay more than half their support, and they have less than $4700 (for 2023) of taxable income ($5050 for 2024). They don't have to live at home or be disabled.
The only real tax difference between the two dependent categories that I know of, is that if you pay for day care to care for your disabled child while you work, you can claim the child care credit if the dependent is a qualifying child but not a qualifying relative. [Added: A qualifying child dependent also makes you eligible for the Earned Income Credit if you otherwise qualify, but not a qualifying relative dependent.]
In Turbotax, there will be questions about the person's age, residency, support and disability that should lead to the right answer for your situation.
