A child must be unmarried and under 21 to qualify as a derivative, but the Child Status Protection Act can freeze the child's age at the petition filing date for certain periods while the petition is pending. Whether the freeze protects this child depends on filing and visa-availability dates, so the calculation should be done before filing.
Run the CSPA arithmetic before the child's birthday
Hypothetical example: the family's daughter turns 21 in fourteen months and is in the middle of a degree. Under the Child Status Protection Act, the time the investor petition is pending is subtracted from her age when a visa becomes available, and she must seek to acquire the visa within a year of availability. If the petition is filed promptly and processing time exceeds the months until her birthday, she may remain a child for immigration purposes; if the family delays filing, or if visa availability for their category lags, she may age out and lose derivative eligibility.
The analysis uses official processing and Visa Bulletin data at the time and should be redone as those change. If she ages out, she is not without options, but each involves her own basis, such as a student status, rather than the family petition. The spouse and younger children are straightforward derivatives, admitted as conditional residents with the investor and included in the same I-829 filing.