According to my calculations, kids in Ontario would be starting 12th grade in the calendar year they turn 17 but kids who have a birthday late in the year or have had problems at school may have been held back a year so surely there are kids there who are older.
The only thing I could find on the web about it was this question on yahoo answers: http://ca.answers.yahoo.com/question/index?qid=20100819054559AAgMDr4
There is no such thing as max age for University or College in North America, this is North America, not Asia, India or China or any asian countries where is there are caps to age in Universities.
There are also adult education for those who have not completed high school or who did not have any qualifying high school results to enter college or universities. Those who do not have, can take adult education, GED adult education to complete the requirements to enter college or universities.