Ordinarily, if the child isn't continuing her education, child support ends at 18, because the child, once out of school, is considered "emancipated," able to support herself at least in theory. Child support does, however, often continue beyond 18, for children in college, although for on-campus students the obligation changes to a portion of the costs and expenses of college, rather than regular payments to the other parent.

Your wife, though, should talk to a lawyer, to see if the court would grant a motion to extend child support, if your step-daughter's handicap means she can't support herself.  This isn't the usual case.