My child was 17 the entire year and turned 18 on December 5, 2024. Why am I not getting the full tax credit of $2000?

The Child Tax Credit is only for a child who is under the age of 17.  When the child is a dependent and age 17 or older they are only eligible for the Other Dependent credit of $500.

You lose the child tax credit in the year the child turns 17.   After that you get the $500 credit for other dependents instead.  

https://ttlc.intuit.com/turbotax-support/en-us/help-article/taxation/additional-child-tax-credit/L6xFeMFEf_US_en_US?uid=lqnuygah

https://www.irs.gov/help/ita/does-my-childdependent-qualify-for-the-child-tax-credit-or-the-credit-for-other-dependents

The child tax credit is only for qualifying children under the age of 17.

Please read this TurboTax article for more information.

@mmwest11  And actually you should have lost the Child Tax Credit last  year in 2023 when they turned 17 .  Has to be UNDER 17 on Dec 31.   Check your 2023 return and see if you got the CTC or just the $500 other dependent credit.