Dependent classification is available to unmarried children under twenty-one, so it simply stops being available. Continuing to live in the United States after that requires a basis of the child's own, assessed on the child's facts. Because that assessment and any resulting application both take time, it belongs on the calendar long before the birthday arrives.
Give the older child a separate file
Open a file in that child's name covering education history, qualifications, employment intentions and any prior U.S. status, and have it reviewed independently of the parents' case. Younger siblings can be planned alongside the principal; the one nearing twenty-one cannot.
Discuss with the child what they want, since studying, working and returning home lead in different directions. Whatever is decided, the household should know the answer before it is forced. Involve the child in the conversation early, because the alternatives are largely about what that person wants to do rather than about the parents' business.
Studying, working, and returning to Canada each carry different requirements and different preparation, and a nineteen-year-old told at twenty-one that a decision has been made for them tends to make a worse decision than one who has had two years to think. Whatever is chosen, the practical test is the same: is there a basis of the child's own, and is there enough time before the birthday to establish it.