Because he is 17. To get the larger credit, like you received for your daughter, your child must be under 17. You should still receive a $500 credit for him though. Child Tax Credit
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This comes as a big surprise to many parents the year their child turns 17. A child over age 16 no longer qualifies for the Child Tax credit (CTC). Although a child can still be a student dependent through age 23, and a qualifying child for EIC, the Child Tax Credit expires the year they turn 17 and you no longer get the $2000 CTC. Instead you will get the non-refundable (up to) $500 Other Dependent Credit.