Family terms are set by the current official framework and must be read from the official source. Do not assume the derivative rules of any other category apply.
Read the family terms, do not infer them
The employment-based immigrant categories include a spouse and unmarried children under twenty-one as derivatives, and it is natural to expect the same pattern elsewhere, but that expectation is an assumption rather than a finding. The official material governs who may be included and on what basis, and the terms have their own structure and their own cost implications. For a couple whose children are adults, this point may determine whether the route is useful at all, so it should be confirmed explicitly and in writing before any commitment. If the answer is that adult children are not covered, their own routes are a separate analysis that should start on its own facts, based on each one's own work, study or investment plans.
Hypothetical example: A Canadian couple hopes to include an adult son who is finishing an engineering degree. The first review should list every proposed applicant, relationship, age, and intended benefit, then compare those facts with the current official family terms. Civil records settle relationships and dates; a broker’s assurance does not. The couple must decide whether the route is still useful if the adult child requires an independent plan. Do not assume that a framework described alongside immigration categories uses the same derivative rules.