Read them in the official material directly. Do not assume the derivative rules familiar from the investment categories, such as unmarried children under twenty-one, carry across automatically.
Ask, do not transfer assumptions
Families who have researched the investment categories arrive with a mental model of derivative eligibility and apply it to anything new, which is unsafe here. The treatment of family members under this framework is governed by its own published terms, including whether family members are covered at all, whether any additional amount applies to them, and how their status is evidenced. Read those terms directly and record what they say and when they were checked.
Where a household's plan depends on a particular treatment of a child approaching adulthood, obtain written confirmation before committing funds, since a transfer of this kind cannot be reversed once the answer turns out to be different from the assumption. Hypothetical example: a small-batch soap manufacturer is considering a gift payment while a child is close to adulthood. The first review should produce a family matrix listing each person, the exact official term said to apply, the evidence needed, and the consequences if family coverage is unavailable or costs differ.
A current official answer, not an analogy to another category, settles the family point. Obtain it before transferring funds and before making school or employment commitments. The familiar failure is assuming that a spouse and children follow automatically because that is common elsewhere.
A new framework may use different definitions or procedures, so a household should treat an unanswered family question as a financial and practical risk rather than a detail for after payment.