Only if she still meets the derivative definition of an unmarried child under 21 when it counts. Because immigrant processing can take considerable time, a child close to the limit needs careful attention, including whether any statutory age-calculation protection applies to her case.
A derivative’s eligibility is measured against a moving process
Start with two dates: the child’s birthday and the realistic timetable of the petition and any visa availability. Hypothetical example: a family from Saint-Cyrille-de-Wendover files while their daughter is twenty years and nine months old, assuming her age at filing settles the question; whether she remains eligible depends on how the applicable age calculation works in her circumstances, and it is a question to put to counsel in writing at the outset rather than to discover at an interview. Marriage ends derivative eligibility outright regardless of age, which is worth telling a young adult before wedding plans are made.
Two further points belong in the same discussion. Each family member who obtains conditional residence has their own two-year conditional period and is included when conditions are removed, so nobody can be added quietly later. And permanent residence is not citizenship: naturalisation is a separate process with its own requirements, and a child who arrives at nineteen does not become a U.S.
citizen by living there.