The investor's spouse and unmarried children under twenty-one may be included as derivatives, with the published age protections applied. A child who ages out needs an independent route.
Ages are measured against a moving timeline
Derivative eligibility is defined by relationship and age, and the age question is complicated by the fact that these cases take years. Statutory age protections exist and are explained in the published guidance, and how they apply depends on filing dates and visa availability rather than on the birthday alone.
For a family with a teenager, that arithmetic should be done at the outset and revisited as the case progresses, because the consequence of getting it wrong is that one member of the household is left without a route while the others proceed. Visa availability by country of chargeability is published monthly and affects the timeline directly, which means the age analysis is not a one-time calculation.