Her age is assessed under the Child Status Protection Act, which for EB-5 freezes her age during the petition's pendency and again depends on visa availability. Whether she stays under twenty-one is a calculation, not an assumption, and it should be done before the petition is filed.
Run the age calculation before you rely on it
A derivative child must be unmarried and under twenty-one, but the relevant age is not simply her birthday on the day she immigrates. The Child Status Protection Act subtracts the time the investor petition was pending from her age at the point a visa becomes available, and the result must be under twenty-one; she must then seek to acquire status within one year of availability. For a Kent family whose daughter is twenty and whose petition may take well over a year, the arithmetic can go either way depending on processing time and the visa bulletin for the applicable category and country.
Marriage ends her eligibility regardless of age. A regional-center project in a targeted employment area may fall into a set-aside category with different visa availability, which affects the calculation. The investor's spouse faces no age question and receives conditional residence on the same terms, including the Form I-829 obligation, filed in the 90 days before the second anniversary.