From my years working in the actuarial field, whenever a rule change such as changing the age ratio was introduced, it was done on a revenue-neutral basis. That is, changing the age ratio from 3:1 to 5:1 could not, in the aggregate, generate more or less revenue than before the rule change. We often filed revenue-neutral rule changes with state insurance departments and they were always approved unless the state insurance department disagreed with the underlying rule change itself.
In order to accomplish that, there would be rate offsets to ensure that the rule change was revenue-neutral based on a given book of business. The amount of the rate offset would depend on how many people were in each of the rating categories and how much revenue each subgroup generated.
Using your $180 and $540 rates for younger and older people, let me first demonstrate 2 extreme examples. Suppose you have 10 people, all age 20-30. To ensure revenue-neutrality, the rate for the 50-60 class would increase to $900, as you wrote. But this assumes that nobody will be in that age class so the total revenue before and after would remain at $1,800 ($180 x 10). The other extreme would assume that all 10 people are age 50-60. There, the rate for the 20-30 would drop to $108, to assure revenue-neutrality ($540 x 10, or $5,400).
But neither scenario reflects the actual population, so what will happen is the rate for the age 20-30 will drop a little while the rate for the age 50-60 will increase a little, creating a 5:1 age ratio while generating the same overall revenue.
Suppose there are 5 people in each age group. Without going through all the algebra, to be revenue-neutral, the rate for the younger group would drop 33% to $120 while the rate for the older group would rise 11% to $600. The total revenue generated would be $3,600 before and after the age ratio change.