Short answer: The ACA caps the oldest adult’s premium at three times the youngest adult’s for the same plan, so coverage is relatively cheaper for older workers and pricier for younger ones than raw cost would suggest.
The 3:1 age band means an insurer can’t charge a 64-year-old more than three times what it charges a 21-year-old for the same plan, even though older members’ claims cost much more. This compresses the age curve, so younger employees effectively subsidize older ones compared with true actuarial pricing. New York and Vermont go further, banning age rating in favor of community rating.