My son was 16 years old for most of 2023. He turned 17 in November. Why am I not getting the child tax credit?

An individual’s tax age is the age at the end of the year. 

To be eligible for the child tax credit the child must be under the age of 17 on 12/31/2023.  Since the child is age 17 in 2023 then you are eligible for the Other Dependent credit of $500.

Sorry but no CTC if your child was not under the age of 17 on 12/31/2023. 

Remember, you get the credit the year they are born, even if they were born at the end of December.  It's not partial or reduced.  So every child is eligible on 17 tax returns, whether they turned 17 in January or the end of the year.