if it is only for phone capabilities, then go for the older model but if there are issues with vision, look for a 6 plus. The larger screen will help.
Experiencing things with my father, he went from a flip phone to an Android about three years ago but at that time he only wanted to check the weather (for fishing) and make calls. When he realized he could do so many other things with the phone, he is burning through data quickly. He hardly uses his home computer anymore. It is amazing to see how quickly they adapt to a smartphone.
One other thing with any phone you obtain and if it is available, make sure they are connected to wifi. it will help in the long run.