So I think we agree on our observation that when someone shows-up to buy an example policy (say Humana Plan G medigap policy) which is registered as an attained age pricing policy, the younger the buyer, the lower the price.
The pressure to close the book is indeed present whenever the attained age pricing methodology is used. This is especially true if the cohort is smaller and an unexpectedly large fraction of the cohort starts having high utilization. That, along with the company realizing they are no longer competitive, but still want to sell more policies to that age group.
But back to the point that the price will be cheaper for younger people for attained age pricing. How does the insurance company decide how much more to charge older people? I think they probably look at last year's claims from each age group and make sure they set the price for next year's policies such that the expected claims will get paid (plus overhead and profit). So we're back at having everyone who has "attained a specific age" being in a separate "group" (cohort). So although the insurance company could stay profitable by allowing, say, younger people to pay more so older people could pay less, I don't think that's what they typically do. If they could get away with it, they'd probably charge the older group MORE (because they can't pass underwriting and would be uninsurable if they left) and use a low price to get lots of young healthy people in the door. But I don't think they're allowed to do that. Each insurance company has to open their financial books to each state's insurance commissioner and justify their actions, I think. So if the insurance company says they're offering an attained age policy, they probably need to follow specific guidelines, and I suspect that means using expected claims data by age to set prices.
I'm no expert at this, and could be wrong about it. If so, I'd be happy to have someone clear-up any misunderstanding I might have.