I am preparing my 2018 tax return. My son's B'day is 10/26/2001. His age is 17 years for tax return 2018. I am getting only $500 child tax break. why ? In 2017 it was 1K

[VolvoGirl]

No.  You are getting the Other Dependent $500 credit.  You lose the Child Tax Credit when they turn 17.  They have to be under 17 on Dec 31.  

[Bees]

To qualify, a child must have been under age 17 (i.e., 16 years old or younger) at the end of the tax year for which you claim the credit. The child must be your own child, a stepchild, or a foster child placed with you by a court or authorized agency. 

https://turbotax.intuit.com/tax-tips/family/7-requirements-for-the-child-tax-credit/L3wpfbpwQ