For marketplace plans, family premiums will reflect the composition of family members, their ages and their tobacco use. To compute a “family premium,” insurers will add together a separate premium for each adult age 21 and older. In addition, insurers can charge a separate premium for up to three children under age 21. In your example, your family premium will reflect three adult premiums and three child premiums. Outside the marketplace, not all plans must follow the same rules for family premiums. (45 C.F.R. § 147.102).