Treat them as separate cases immediately. Derivative eligibility covers unmarried children under twenty-one, with published age protections whose operation depends on filing dates and visa availability.
Different children, different clocks
A household with children close to the age boundary is really running two or three processes rather than one, and the arithmetic should be done before anything is committed. Derivative status covers the spouse and unmarried children under twenty-one, and statutory age protections exist whose operation depends on filing dates and on visa availability rather than on a birthday alone. For a child already past the boundary, an independent route has to be identified, whether that is study, employment or something else, and it needs its own timeline. For a younger sibling, the calculation should be revisited as the case moves, since the monthly bulletin's cut-off dates for the applicant's chargeability can shift the timeline substantially in either direction.
Create separate entries for each derivative’s age, passport, relationship document, and intended travel path. The relevant immigration records settle classification, while the Visa Bulletin and case progress may affect timing independently of the family’s readiness. The decision is whether everyone can proceed together or whether an independent option needs early attention for a child nearing twenty-one. Hypothetical example: a violin-restoration business owner has a daughter approaching the age limit, so the family evaluates a separate student plan before making an investment decision that cannot accelerate visa availability.