Customize interface for elder / older users My father in law asked me if a "elder smartphone" is good for him, since he is having difficulties with an old iPhone his wife gave him. After digging a little bit I discovered that the "elder smartphone" being sold is nothing more of an Android with a better launcher with bigger font and easier to comprehend icons.

I've done some searching here and only found this answer below:
Is there a larger interface for older owners?

Which is from 2014.

The question is: Do you guys have any experience with launchers designed for elder people ? What would be the do's and dont's that such launcher should have ? Are there any native settings that would help him to use an Android smartphone better and easier ?

His basic use is to send/receive SMS, place and receive calls and use WhatsApp. No games, no need for a fancy camera. A large battery would be nice too.
You don't need a phone specifically geared to "elderly people", as you will likely just end up paying more for less just to get that tag.

The latest Android N (Nougat) version offers a quite advanced Screen Zoom function that can enlarge all screen items. You can see it in action here.

On older Android versions, you mostly have the option of setting text size, and if you have root or install a custom ROM like Cyanogenmod (which in its official versions is usually totally unproblematic for daily use even by non-tech-savvy users), you can change the device's DPI setting, which basically lets the system know about the physical screen size of the phone. 

By setting the DPI value higher, the system believes your screen is smaller than in reality and makes both text size and icons larger (in a limited number of increments, not continuously). I have found in the past that this works quite well (using it for instance to enlarge all items, including map scale, in navigation apps).

So I would recommend choosing a well-reviewed large-screen phone (the largest your father-in-law cares to pocket) with Android N software (or with Cyanogenmod support) and enlarging the screen items as detailed above.