Short answer: Age (member-level) rating charges a separate premium for each person based on age; composite rating charges the same tier rates for everyone regardless of age, using a blended average.
Under age rating (member-level rating), each enrollee’s premium is set by their age using the ACA age curve, and the group’s total is the sum of those premiums. Composite rating instead sets uniform rates by tier, for example employee-only, employee+spouse, family; that don’t change as employees age during the year. Composite billing is simpler and can favor groups with an older workforce, while age rating can be cheaper for groups with many younger employees.