Incompetent people can still have taxable income. To use a current example, the comedian Tyler Perry is legally incompetent from injuries suffered in a car accident. However, I'm sure that he's still receiving millions in residuals and has substantial assets that produce income. If your daughter had taxable income, her guardian needs to file a return and pay the tax out of her assets. Most state income taxes don't have a medical expense deduction, which might eliminate her Federal taxes.
Answered on Jul 20th, 2015 at 12:57 AM