It's true. If an insurance company wants to charge 50X, they can. Of course, with a functioning market those insurance companies would promptly collapse because the entire senior sector would take a hike to a more reasonable company advertising more reasonable rates. Having Uncle Sugar setting the rates short-circuits that market effect.
Obviously, they wouldn't charge 50X. They'd charge whatever multiple is actuarial sound. For example, pre-ACA, a 27 year old person would pay $117/month for a policy that would cost a 60 year old person $735/month. That's a multiple of about 6.3
http://khn.org/news/age-rating/The ACA capped that multiple at 3, and the AHCA capped it at 5. So if you repeal the ACA without replacing that provision, you could expect prices for older people on the individual market to see an increase to pre-ACA levels - a 6.3 multiple.
Of course, you're right -- if a company tried to jack up that multiple
above the actuarial sound rate, older people would flee to companies that charge a more reasonable rate, and those other companies would make more money. But on the flip side, if a company charged
less than that actuarial sound rate -- say, 3.0 without a government mandate, then older people would flock to
that company. But that company would then be taking in much less in premiums from older people than it was paying out in claims, so it would have to jack up premiums for everyone
younger. Those younger people would then start moving to plans that were cheaper for them, and the company charging below-cost for older workers would go belly-up.
Bottom line is that in the absence of a government mandate limiting what insurers can charge on the open individual market, the repeal and non-replacement of the ACA would result in costs for older workers more than doubling -- from the ACA multiple limit of 3, to the market rate of over 6.
I still support moving to the freer market. But it seems as those some of those advocating for a complete repeal want to have their cake and eat it too -- repeal, but retain the non-market based restriction on how much older people can be charged on the open market.