For 2020 it came up when filing not eligible for CTC (he is 17). I've gone through each eligibility criteria's and it doesn't seem to exempt him from being a dependent and eligible for CTC. No shared filing with anyone.
A child must be under age 17 to qualify for the child tax credit. A child qualifies you for the CTC if the child meets all of the following conditions.
The child is your son, daughter, stepchild, eligible foster child, brother, sister, stepbrother, stepsister, half brother, half sister, or a descendant of any of them (for example, your grandchild, niece, or nephew).
The child was under age 17 at the end of 2020.
The child did not provide over half of his or her own support for 2020.
The child lived with you for more than half of 2020 (with some exceptions).
The child is claimed as a dependent on your return.
The child does not file a joint return for the year (or files it only to claim a refund of withheld income tax or estimated tax paid).
The child was a U.S. citizen, U.S. national, or U.S. resident alien.
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