US citizens can't use Medicare when living abroad.
Also, when US citizens start getting their SS payments, I believe that certain medicare part D premiums are automatically deducted from their SS payments. This would be quite bad if the US citizens in question can't use Medicare at all. However, there might be a way to avoid having these Medicare part D premiums getting automatically deducted from SS payments.
You guys are younger than I am (I'm 53), but you might want to start thinking about all this ....
And all US citizens expats out there, feel free to educate me further on this!