The 19-year-old may, as an L-2 child, until turning 21. The 22-year-old is not a derivative and would need a separate status, such as a student visa or an employment category of their own.
Twenty-one is the line, and it is not extended
Hypothetical example: a Frontenac Islands manager transferring to the New York yard has a daughter of 19 finishing college and a son of 22 who helps on the farm. L-2 status is available to a spouse and to unmarried children under 21. The daughter qualifies now and may study in the United States in L-2 status; when she turns 21 she loses derivative status and must have changed to something else, such as F-1 if she is enrolled, before that birthday.
The son is already over the limit. He might work for the U.S. yard in his own right only if he qualifies for a category himself, and neither L-1 nor TN fits an employee whose experience is farm labour.
He could visit as a visitor, without working, or remain in Canada to run the home farm, which is often the better answer for a family business. The petition should list only the eligible dependants and the family should plan the son's role separately. Hypothetical example: A Frontenac Islands wholesale-food company transfers its operations director, whose spouse plans to seek local accounting work and whose child is twenty.
The family should first confirm each person’s age and relationship with passports and civil records. An L-2S spouse’s admission record is the document an employer will examine for work authorization; the twenty-year-old may study but must not work on that dependent status. A child who will turn twenty-one soon needs an independent plan before the move.