When does medical insurance stop for a child insured by the non-custodial parent's coverage if the child is 18 but a full time college student?

as long as your child continues to be a full time student, then they are covered until they graduate. That may not be the case. Most insurance will cover a student to age 21 - 25. So if the student wants to pursue a doctorate that will take them past their cutoff date then they will be terminated. It depends on the policy.