Short answer: Under age rating, premiums rise with age up to a 3:1 cap, so older employees have higher individual premiums. A percentage-of-premium employer contribution helps offset that by paying more, in dollars, for them.
With age (member-level) rating, each employee’s premium reflects their age along the ACA curve, where a 64-year-old can be charged up to three times a 21-year-old. That makes coverage pricier for older workers on paper. Employers can soften the impact through their contribution design: a percentage-of-premium contribution automatically pays more real dollars toward older employees’ higher premiums, while a flat-dollar contribution does not. In community-rating states (NY, VT), age doesn’t affect the premium at all.