My mother who has severe issues maintaining her oxygen levels and has to be on oxygen while she sleeps wears a mask while she's out and about fine.

I'm guessing the issues people face is less about the masks and more about the sensation of wearing a mask which spikes anxiety levels and causes the sensation of being unable to breath. If Japan can reach near ubiquitous levels of mask wearing without issues especially given their larger elderly population, I'm sure they'll be fine.

Otherwise if they have an actual medical issue that prevents wearing a mask then by all means.