I am not sure about other countries, but in Portugal there are some mobile phones (usually not smartphones) which are catalogued as for elder people. As you can see above, they have big buttons, with large numbers. But things stop there. If you try to use any of them, you will notice that the text shown in the screen is quite small (not talking about the current time, as shown in the images). For example, it is quite hard for someone with impaired vision to read a message. One can argue that elder people will not use SMS. But if that is the understanding of the phone makers, just remove that functionality from the phone. In the other hand, if you think elder people will read and/or send messages, then, allow him to configure the font size.
Please, the first that does that, send me a message and I will buy one for my mother.