A spouse and unmarried children under twenty-one may be included as derivatives, so an unmarried twenty-year-old may be, subject to the statutory age calculation that applies while a petition is pending. A married child cannot be included at any age, and marriage after filing affects eligibility.
Work the ages against the filing calendar
Age is a moving target in this process because time passes between filing, approval and the grant of conditional residence. There is a statutory calculation that can preserve a child's eligibility for part of that period, and it depends on dates the family does not control. The practical response is to identify the exact date of birth and marital status of each child at the outset, take advice on the calculation for that child rather than a general rule, and avoid delay that is within the family's control, such as slow document gathering.
Discuss openly with an adult child what happens if they age out: whether they would study in the United States on a student visa, remain in Canada, or pursue a route of their own later. That conversation is easier to have at the beginning than at the end. Hypothetical example: a diagnostic-laboratory investor has a twenty-year-old unmarried child who plans to marry after graduation.
The timing of family changes may be as important as the investment documents. Prepare a family table listing marital status, dates of birth, passport details, and the intended timing of each stage. A first review should identify who may derive status and who needs another plan.
The usual mistake is assuming a later family event can be ignored because the investor's petition is strong; derivative eligibility has its own conditions and deadlines.