Possibly. The Child Status Protection Act freezes her age during the time the petition is pending and requires her to seek to acquire a visa within a year of availability. Whether she stays under 21 for immigration purposes depends on the filing date, processing time and visa availability, so file promptly and calculate carefully.
Her birthday is a filing deadline
A derivative child must be unmarried and under 21, but the Child Status Protection Act allows the time the standalone investor's Form I-526 petition was pending to be subtracted from her age when a visa becomes available, provided she seeks to acquire residence within a year. Hypothetical example: a Lake Country contractor's daughter is 20 years and 4 months when he files; if the petition takes 18 months to approve, her age for immigration purposes is calculated by subtracting those 18 months from her age at the time a visa number is available, which may keep her under 21 if the visa is current then. If EB-5 visas for Canadians face a backlog in the Visa Bulletin, the calculation can go the other way.
The safest course is to file as soon as the case is ready and to check the Visa Bulletin monthly. If she ages out she will need her own basis, such as a student visa, and could be sponsored later as an adult child, on a much slower timeline.