Her age under the Child Status Protection Act is frozen while the petition is pending and then continues to run until a visa is available and she seeks it within a year; whether she stays under 21 depends on those actual dates, not on the filing alone.
CSPA subtracts pending time; it does not stop the clock
Hypothetical example: A derivative child must be unmarried and under 21. For a Portugal Cove-St. Philip's family the Child Status Protection Act helps in a specific way: the time the EB-5 petition was pending is subtracted from the child's age on the date a visa becomes available, provided the child seeks to acquire the visa within one year of availability.
A daughter who is 19 at filing, whose petition takes two years to approve, and whose visa is available at approval has a CSPA age of 19 and qualifies. If the category is backlogged so that a visa is not available until she is 23 and the pending period was two years, her CSPA age is 21 and she does not. The calculation uses the Visa Bulletin chart that USCIS designates for that month, which changes.
Bring the exact birth date, the intended filing date and the current bulletin; the answer cannot be given from the filing date alone. A child who ages out needs an independent route.