At age 21, no one, not even a disabled child qualifies for the Child Tax Credit (CTC). Adult dependents and "qualified child" dependents, over age 16, qualify the taxpayer for the $500 Other Dependent Credit. This credit is essentially a replacement for the old dependent exemption deduction [~$4000].
The credit begins to phase out at $200,000 of modified adjusted gross income. This amount is $400,000 for married couples filing jointly.
If your son qualifies as your dependent, he is not eligible for the stimulus payments, even if you choose not to claim him. He can also not claim the refundable portion of the education credit.
