At what age does child support end? Is it 18? Or is it when the child completes school up to age 23?

Yes, yes yes and sometimes no.

It will end when it says it ends in the child support agreement that was written up and/or modified over the years.

If it states it will go though college till ( some age) then that is what it does, if it states it ends at 18 then it will end then. If it does not say, the one paying support at 18 will go and ask for it to end