While your son was 16 for most of the year, the IRS determines eligibility for the Child Tax Credit based on your child's age at the end of the year (December). Since he turned 17 in September, he no longer meets the "under age 17" requirement for the full $2,000 credit regardless of how much support you provided. However, you can still claim the Credit for Other Dependents, which is worth $500, because he still qualifies as your legal dependent.
