Being disabled for tax purposes has to meet this IRS requirement -
Permanently and totally disabled.
Your child is permanently and totally disabled if both of the following apply. • He or she can't engage in any substantial gainful activity because of a physical or mental condition. • A doctor determines the condition has lasted or can be expected to last continuously for at least a year or can lead to death.
A child's disability doesn't affect tax filing until he turns 19 (24 if a full time student). Then his disability allows him to continue to be considered a "Qualifying Child" for tax purposes regardless of age. There are no credits or deductions, other than that some items, e.g. special schooling, can be considered medical expenses.