Ontario: In Ontario for non divorce cases, the Family Law Act says that it is up to age 18 if the child is enrolled "in a full time program of education" or even at age 16 and 17 if the child "has withdrawn from parental control".
Divorce: If the case includes divorce, then the Divorce Act time limit can be much later. If the child is over the age of majority (18 in Ontario), then child support can be ordered where the child is "unable, by reason of illness, disability or other cause, to withdraw from [the parent's] charge or to obtain the necessaries of life."
Factors outlined: A 2012 Divorce Act case - Szitas - presents a very nice outline with respect to some of the factors that should be considered.
What about the situation where the child who is 18 or older totally rejects the payor parent? Should that parent have to continue to pay child support? Should the rejection of the parent perhaps influence the amount of support that should be paid? Szitas contains a useful reference to some of the leading case authorities. Bottom line?: The termination must be a unilateral and unjustified act on the child's part. Secondly, this factor will rarely be the clincher issue and where it is, it is more likely to affect the final amount than the actual liability to pay child support.
Education Program: The issue of enrolment and active participation in an educational program provides much more fruitful grounds to actually terminate child support. Szitas contains a very useful summary list of factors to examine (see paragraph #37 of the decision).
The 2012 court decision - Smith - takes a hard look at the issue of whether the adult child was seriously engaged in college education. Justice M.S. James decided that the "child" in this case just did not make the grade in that respect:
Robert enrolled at Algonquin College in September 2007 just after his eighteenth birthday. He registered in a two year program and ordinarily would have graduated in April, 2009. In his first semester Robert failed four of five courses. In his second semester he failed all five courses. He did not re-enroll until the second semester of the following year, being the semester running from January to April, 2009. He failed the four courses he enrolled in. He was then absent from school and working full time for at least part of the time from May 2009 until January, 2011, a period of about twenty months. He re-enrolled on January 4, 2011 and failed two of four courses. During the fall, 2011 semester commencing September, 2011, he failed four courses.
Remember though - each case is different. There are many facts and interpretations of those facts that could impact your situation.