Good advice from Svim. I'll add this in case it helps...
My 80+ year old parents live on the opposite side of the country, but I gave them Galaxy S5s as their daily-driver phones (without doing anything special to the phones), and they're doing fine. The S5 has an "Easy Mode" option in the settings so that you can put a few, big apps on the home screen, but I didn't use it.
I set-up the phones for them, installing and setting up apps that I thought they'd like (like Weatherbug, Netflix, & WatchESPN). I set up their email accounts, calendar, contacts, & favorites (which sync to their PC) and I arranged the desktop. I wrote them a user manual for the key functions-- which I think really helped because they could refer to it over and over until they learned. They've been doing just fine with their phones. My 82-year old mom is a whiz. She lives on the thing for email, browsing, texting, eBooks, etc. My 84-year old dad is proficient enough. He really took to it when he finally grasped the convenience of talking to the phone using Google Now commands and voice-to-text synthesis for email/text. Shocking, actually.
I strongly suggest installing TeamViewer "Host" on grandpa's tablet, and TeamViewer full version on your PC/Mac/Android (and connecting them once before you hand over the tablet). Then you can remotely drive grandpa's tablet anytime that it's online. You can remotely change or fix stuff, or maybe give him that occasional lesson.
I installed "
Autosync Google Drive" on their phones and set it up so that if they take any photos/videos, they'll be automatically uploaded to a "photos" folder inside their Google Drive folder (which is on their PC's desktop and mine). That way they don't have to figure out how to transfer photos to their PC and they don't have to figure out how to send them to me.
