She can be included as a derivative if she remains unmarried and is under 21 when it counts. Age is measured under the statutory calculation that can subtract certain processing time, so her position should be assessed specifically, not assumed.
Nineteen is close enough to plan around
Hypothetical example: a Grand Bay-Westfield family assumes their daughter is safely inside the age limit because she is nineteen when they start. EB-5 processing can run for years, so that margin is smaller than it looks. Derivative status is available to the spouse and to unmarried children under 21, and the statutory age calculation may allow certain pending time to be subtracted from a child's age, which sometimes preserves eligibility for a child who has passed 21 in ordinary terms.
Whether that helps depends on the dates in her particular case. The practical steps are to file without avoidable delay, keep evidence of every filing date, avoid gaps caused by incomplete documents, and identify an independent route for her — a student status, or her own employment category — so that a delay does not leave her without options.