What Does Birthday Rule Mean?
The birthday rule states that when a dependent child is covered by the health insurance policies of two parents, the primary policy for the child will be the one held by the parent whose birthday is the closest to the beginning of the year.
Insuranceopedia Explains Birthday Rule
Under the birthday rule, it doesn't matter which parent is older; it only matters whose birthday is closest to the beginning of the year.
The rule exists because a standard needed to be set for determining which insurer has to be the primary insurer for dependent children. Although somewhat arbitrary, the birthday rule prevents a protracted settlement process by insurance companies who wish to avoid being the ones to pay the claims.