It ends her eligibility as your derivative. Only unmarried children under 21 may accompany an E-2 principal, and because E-2 offers no direct path to permanent residence, a child who ages out cannot simply be carried along until a green card arrives.
A birthday is a deadline, and this category has no bridge
Plan for the child who is closest to the limit first, since that date drives everything else. Hypothetical example: a couple from Saint-Cyrille-de-Wendover are buying a millwork installation company in Massachusetts, with children aged seventeen and twenty at the time of filing; the younger has years of derivative eligibility, while the elder will be outside the definition within months of arrival and needs her own answer well before then. Her realistic options are her own status in her own right — a student status if she will study, or a work-based classification if she qualifies for one — and each has a lead time measured in months.
The spouse’s position is more comfortable: an E-2 spouse is employment-authorized incident to status and may take a job with any employer, subject to any professional licence the work requires. Derivative children may study but may not work. Write the ages, the dates and the intended activity for every family member on one sheet before the purchase closes.