Child tax credit, please explain

Im not sure whats the best group for this. So my daughter will turn 17 on December 30th, and I was just told that means I can't claim the child tax credit for her on my 2020 tax return. Im like wtf?? I mean she is in 11th grade and I have taken care of her like my child (since she is) all year. But because she turns 17 one day before the end of the year, she's automatically dismissed as a child!? So the rest of the year leading up to the 30th doesn't count? This is just new to me, I thought as long as you took care of your child for over half of the year, they would qualify.