Do I have to pay child support after the child turns 18 if we created an agreement saying payments would be stopped then?
Under Canadian law, there is one basic principle that applies to domestic contracts of all types, especially when they deal with any children you have together: A court can vary your agreement, and even impose brand-new legal obligations on either or both of you.
So if you and your ex have agreed that child support payments will cease once your child turns 18, a court can still override that decision, based on the best interests of your child, and rule that child support should nonetheless continue to be paid by one or both of you.
The obligation of every parent to support a child is enshrined in legislation, and for all parents that law can impose a duty to pay even after the child reaches the age of majority in certain circumstances, most commonly where the now-adult child has embarked on a course of post-secondary education and remains financially dependent. Needless to say, whether this will apply to you can depend on a host of circumstances.