Is it legal to have to pay child support for children after age 18 who are not seen or visited?

 Generally speaking, if your children have decided not to pursue higher education at a post-secondary institution, then your legal obligation to support them will generally end when they are 18 (unless there is another reason for their dependency).  In some relatively uncommon circumstances, the court may condone a child taking a relatively brief, reasonably-justified break from school, and the parents to continue paying support afterwards, even despite the break.   However, if your adult child has inexplicably or unreasonably waited years to return to school, the court will be unlikely to impose a support obligation on you.

As for the fact that you have no involvement with your children:  There have been some Ontario court decisions that confirm that – provided the child meets the threshold legal tests for support entitlement – a parent may still have a financial obligation even if they do not have a close (or any) relationship with their child.  However, the court will look at all the factors; especially if your child is well into early adulthood at the time of the request and is otherwise independent, it is unlikely that you would be asked to pay for their schooling in any event.