A spouse and unmarried children under 21 qualify as derivatives. Age is assessed under the rules that govern it rather than on the day the family moves, and the Child Status Protection Act can preserve eligibility in some circumstances, so a child approaching 21 during a long queue needs specific advice rather than an assumption.
The birthday and the queue have to be planned together
For a household with teenagers this is often the decisive issue, and it is far more consequential than any question about the investment itself. Derivative eligibility depends on the child being unmarried and under 21, and the practical difficulty is that immigrant categories can involve waiting, sometimes years, depending on the applicant's country of chargeability. The Visa Bulletin is where those waits are published and should be read at the outset rather than after filing.
Where a wait is long, the Child Status Protection Act may allow certain periods to be deducted from a child's age, which can preserve derivative status that would otherwise be lost, but the calculation is technical and depends on facts that vary. A child who ages out is not left without options, but the options are their own, generally study or employment routes they qualify for personally. Marriage before admission also ends derivative eligibility, which surprises families more often than it should.