A derivative must be an unmarried child under twenty-one at the relevant time. Whether she remains protected depends on the statutory age calculation and on the dates, so it should be assessed specifically.
Do the calculation on real dates
Once a derivative has been admitted or adjusted as a conditional resident, her position is different from that of a child still waiting for a visa number, and the age question is usually settled at the point she obtained that status. Even so, families in this situation should have the position confirmed on their actual dates rather than assumed, because the consequences of being wrong are severe and cannot be corrected retroactively. Marriage during the period is a separate disqualifying event for derivative eligibility. Where any doubt exists, the safest step is a review well before the birthday, so that an alternative can be prepared if one is needed. Alternatives generally require lead time and cannot be arranged in the weeks after a deadline has passed.
Hypothetical example: A conditional resident’s unmarried child is close to twenty-one and is considering marriage while the family prepares its later filing. The first review should collect the child’s birth record, present immigration records, and any event dates that may affect derivative treatment. Those documents settle the timeline, not a family plan to resolve it later. The decision is whether the child needs independent advice or a separate strategy before a status-changing event occurs. Do not assume that the principal’s investment preserves every family member’s position regardless of age or marital status.