There isn't really a set age. It would mostly depend on how comfortable a child feels going to the bathroom without a parent, how comfortable their parents feel about the child being in the restroom alone, and, of course, how well the child is able to use a toilet by themselves.
do u have to pay child support when the child is still in school at the age of 18
Even so, the child is still a minor and you are responsible for her until she is of age.
Yes, but generally not past age 23.
Until the child has reached age of majority in their state, usually 18.
Child support continues as long as the child is still a dependent; this includes school and university, regardless of age.
see link below
no 21 is the cut off age
this depends on what country, in most it is 12
until age 19
Yes, until age 18.
Until age 21, which is the age of majority in the state. see links
If it's past due child support, yes, it's still owed.