Have you tried ChromeOS? My grandfather is in his mid 80's. I've been helping him with his computer(s) for years. In the last year, I switched him over to a Chromebook, after trying and failing to switch him to Windows 10 (wasn't compatible with his old computer)... and since everything he does involves websites and bookmarks anyways.

It's the easiest time he's ever had using a computer. No random update notifications, adware popups, anti-virus subscription nags, etc. Nothing but a browser, which he knows how to use.

I'd recommend it to anyone considered technically illiterate.