In case anyone else is looking for the same info, here's what I've learned:
IMPORTANT INFORMATION about applying for Medigap coverage in ALL STATES: When you first enroll in Medicare, you have the right to join any plan that is offered in your coverage area. You cannot be rejected or charged higher premiums due to a pre-existing condition.
HOWEVER, if you want to join a Medigap plan after your open enrollment period ends, you WILL be subject to medical underwriting, and may be rejected or charged a higher premium.
There are SOME exceptions where you may be granted a chance to enroll without medical underwriting. More information about that can be found here: medicare.com/medicare-supplement/medigap-and-medical-underwriting/
Regarding Medigap coverage in New Jersey:
1) In New Jersey, if you're under age 65 you can only get Medigap Plan C.
2) Few companies offer any coverage under age 65.
3) The companies that do offer underage coverage typically only offer coverage down to age 50. If you're under 50 you're mostly out of luck.
4) Some insurance agents will THINK they have plans for you, because the underage plans will come up when they enter your information. HOWEVER, when they try to put in an application it will be rejected if you are under age 50.
5) As far as I can tell, the ONLY company currently offering Medigap coverage under age 50 in New Jersey is Horizon Blue Cross Blue Shield. In 2018 they offer Medigap plan C for $240.21 per month.
For many people, it may be more affordable to choose a Medicare Advantage plan instead. HOWEVER, if you have numerous hospitalizations/specialist appointments/procedures each year, by the end of the year the medical copays could easily end up being even more expensive than the Medigap premium. Additionally, Medicare Advantage plans are HMOs and will limit you to providers in their network, whereas original Medicare can be used anywhere in the United States.
Also, I did not consider prescription costs in any of this. All the Medicare prescription plans end up having exorbitant copays for CF medications. The only way to bring down the cost is with copay assistance programs, such as the Healthwell Foundation or drug-specific patient assistance programs.
I hope someone finds this helpful!