strong. I don't want to say "failing" just very poor memory and the like caused by very poor health. One case is a man who was in the tech industry years ago, but that doesn't mean that as his mind was harmed by the effects of illness he could continue to make sense of Google's ever changing interface. He didn't have a problem with mail so much (he used thunderbird, so that interface wasn't changing), but he couldn't navigate Google's changing phone service interfaces. Combine that with poor eyesite and problems with phone drivers that occasionally have to reload ... and there would be days when he had no phone service until someone came by and fixed his computer.
Keep in mind that there are people (once again the same man) who at times find simply dialing a phone too hard. Maybe they're too slow for hospital phone that gives you 20 seconds of dial tone then gives up, or worse gives you 20 seconds but no audio cue like a dialtone.
For such people you need interfaces designed differently than ones for average customers. You need interfaces that NEVER change. You need interfaces that have no time-outs. You need interfaces that force modal interactions rather than assuming that the user will NOTICE something.