Up to what age can child support stop if child is over 18 and in college.

At the age it states in the child support agreement. It varies by what nation you live in, US, Canada, Frace, India, we have people here from all over the world. If in the US, it will depend on state, and even the state law at the time of your child custody agreement.
In GA it is currently 20, some states do not require it at all.
But you are obligated to the terms you agreed to in the custody agreement