Family terms, and any additional amounts, must be checked against the official FAQ at the time of application. Under EB-1 and EB-2, a spouse and unmarried children under 21 are ordinarily derivatives, and a child close to 21 needs the age rules examined.
Two sets of family rules, checked in order
Hypothetical example: a Rothesay founder's daughter is 20 and in her final year of university in Canada. The official Gold Card page and FAQ should be read for whether dependants are included, whether additional amounts apply and how family members are vetted; those are program terms and can change. Then the underlying employment-based category's rules apply: a spouse and unmarried children under 21 may immigrate as derivatives, and a child who turns 21 during the process may lose derivative eligibility unless statutory age-protection rules preserve it, which depends on the dates of filing and adjudication and requires its own calculation.
The daughter's own plans matter too: if she intends to work in the United States after graduation, her own route may be quicker or more certain than waiting as a derivative. The family should record each person's date of birth and current status and have the reviewer map both sets of rules against a realistic timeline.