When age is assessed at each stage, and what would happen if a child turns twenty-one during the process. The spouse and unmarried children under twenty-one are the qualifying derivatives.
Ask the question while there is still time to act
The date does not move, and the process takes years, so a household with a child in their late teens should ask the question at the outset rather than as it approaches. The Child Status Protection Act contains provisions that can affect how a child's age is calculated in certain circumstances, and visa availability as reflected in the Visa Bulletin influences the timeline that drives the calculation, so the analysis is specific to the facts and needs advice on those facts rather than a general answer. Alongside it sits a more practical question: if a child does age out, what would their own route be.
Study, their own qualifying employment and other family-based options each require preparation. Knowing the answer before it is needed keeps it available.