A spouse in L-2 status is employment-authorized incident to that status and may work without applying separately for permission, and unmarried children under 21 may live and attend school. A child who turns 21, or who marries, stops qualifying as a derivative and needs a status of their own.
The spouse rule here is the generous one; the age rule is not
Households comparing categories usually discover the difference at this point. Where a TD spouse may not work at all, an L-2 spouse is employment-authorized by virtue of the status itself, which changes what a two-income family can plan for. That authorization is derived from the principal's status, so it lasts exactly as long as Étienne's L-1A does and ends with it.
Children are the harder part of the arithmetic. Derivative status for a child depends on being unmarried and under 21, and both facts are tested continuously rather than only at admission, so a seventeen-year-old at the start of a seven-year maximum stay will not be a derivative at the end of it. Families in that position generally plan the child's own route — a student status, or an independent basis — well before the birthday, because a transition arranged in advance is straightforward and one arranged afterwards frequently is not.