It depends on the specific facts, and nobody can promise the result. Statutory age protection can, in some cases, hold a child's age below 21 using defined periods and the moment a visa becomes available. Have counsel run the calculation on your actual dates rather than relying on a general reassurance.
Two children, two different answers
For each child, distinguish biological age from any protected age under the rules that apply to the case. Collect the petition and visa-availability history and review required actions promptly. A sibling’s result does not establish another child’s eligibility, and an older child may need an independent route if no protection applies.
Retrogression does not itself cancel permanent residence already granted to a family member. Treat the child's age question as the item that sets the household's whole sequence, because it is the only one that cannot be rescheduled. Everything else, from a document ordered to a medical examination to a move, can be brought forward or delayed.
Ask counsel to run the calculation on your actual dates and to say plainly which actions must be taken promptly and what follows if they are not. Hypothetical example: a family invested in a project serving a clinical-supplies wholesaler assumes that a sibling's outcome settles the question for an older child, when the two stand on entirely different footings. Where an older child may need an independent route, the useful moment to identify that is while alternatives remain open, not after a visa becomes available and the remaining choices have narrowed.