The spouse and unmarried children under twenty-one are derivatives. A child's age is assessed at particular points in the process, and provisions exist that can affect the calculation.
Ages move while the process runs
Hypothetical example: A household with a teenager should treat the child's age as a live variable rather than a fixed fact. Unmarried children under twenty-one may be included as derivatives, but the process spans years, and a child who is nineteen at filing may not be under twenty-one at every subsequent stage. The Child Status Protection Act contains provisions that can affect how age is calculated in certain circumstances, and the Visa Bulletin's availability position affects the timeline that drives the calculation.
None of this can be resolved by assumption. A Selwyn family should map each child's birthday against the expected stages, identify the point at which an age problem would arise, and take advice specific to their facts. It is also worth planning what an ageing-out child's own route would be, so the answer exists before it is needed.