Unmarried children under twenty-one may be included as derivatives. Whether he remains eligible when a visa becomes available depends on age-out protections that must be assessed on the actual dates.
Age-out is a calculation, not a hope
Hypothetical example: A derivative child must be unmarried and under twenty-one at the relevant time, and because employment-based immigrant processes span years, a child who is twenty at filing may be over twenty-one when a visa number becomes available. The Child Status Protection Act provides a calculation that can subtract certain processing time from the child's age, and the Visa Bulletin governs when a number is available for the relevant category and country. Neither can be assessed in the abstract; both depend on real filing and approval dates. A family in this position should have the calculation done at the outset, because the answer may influence whether the filing is made now, and what alternative the son should be preparing in parallel.
Review every derivative family member's age, marital status and immigration history at the start, not after the principal's petition advances. Hypothetical example: A water-treatment company investor has an unmarried child nearing twenty-one who expects to study abroad. The household needs to assess whether the child's timing creates a separate immigration concern and keep the relevant civil records current. The business investment itself does not pause a child's age calculation.
Obtain advice on a close age calculation before relying on a tentative timetable. A family decision may need to be made while the investment plan is still flexible.