My child is turning 18. What obligation do I have to continue medical insurance and expenses from medical bills

As far as the insurance part of it it's a moral duty to keep carrying it if your child is beyond 18 and hasn't gotten a job that offers it or is still in school. Medical expenses can be devastating. As far as is it legal to stop it after the child has reached the requirements of a child support order . Yes unless there are circumstances that mandate it like special needs.

Unless of course you have a court order from child custody and support that says you have to prove support though a certain age if they are in college.