Age protection exists but is fact-specific and cannot be promised, so no one should tell you a particular outcome. Confirm his exact date of birth against the case chronology with counsel now. Marriage would end his eligibility as a derivative child regardless of age, which is a decision worth discussing early.
Pending derivatives, and relatives who already hold residence
Build the age review from the actual birth date, petition dates, visa availability and relevant actions under the applicable age-protection rules. Ask how any proposed new filing could affect that analysis rather than assuming an amendment preserves every date. Distinguish the son’s pending derivative case from family members who already obtained permanent residence; later age or marriage does not automatically undo a grant already made.
Derivative inclusion should be distinguished from a separate principal investment case. Eligible derivative spouses and unmarried children under 21, subject to individual CSPA review, have their own immigration evidence needs; family participation does not replace the job requirement. A child's age is calculated under the Child Status Protection Act, but a child close to 21 needs the timeline reviewed before filing. Whether a particular child is protected depends on the filing date, the approval date, the visa bulletin and the child's birthday, and no general assurance can be given here.