My grandparents (both in their 80's) have some iPads, phones, and a Windows laptop as well. It has been interesting learning about how they use them and what specific issues they encounter and how they deal with them.
The Windows laptop was the first thing they abandoned. They like to pick up their devices, do a quick search online, then close them up and put them away again. They may not use the device again for another week or more; longer for the laptop. This meant that they used the laptop so infrequently that whenever they did try to use it - the battery was always dead and they would get absolutely hammered with Windows updates. They would then unplug the machine which would cause it to die (dead battery) when they were done using it for a few minutes, killing it mid-update. Multiply this a few dozen times over a year and you have a very very unhappy laptop and frustrated grandparents.
The cellphones were another major issue. They've had several iPhones and a couple dumbphones/featurephones. The issue with the iPhones were the complexity and screen size. Trying to get an 80+ year old senior to use a tiny touchscreen keyboard is simply out of the question. Only one of them could read the screen and that's because she had Lasik eye surgery. They also kept running into annoying little problems that I would have to fix whenever I visited. For example, they could not grasp the concept of a long-tap and how it invoked additional options. No matter how many times I instructed them, they simply would not lift their fingers off the screen fast enough. I highly recommend adjusting long-tap delay timing if you have elderly family with the same issue but this will vary greatly by device; I don't think the iPhone had it. The dumbphone worked a bit better, especially as we got them one with big chunky buttons they could easily see and press. The problem was that they used it so infrequently that the battery was always dead and they always lost the charging cable and/or forgot which cable was which. To this day, they basically never use it and we've been trying to get them to cancel the phone plan for it because it's wasting them money.
By far the biggest success has been their iPads. They really enjoy sitting in their chairs and browsing the net with the larger, brigther, and clearer screens. I adjusted font sizes to be easier on their eyes and they both find it much easier to navigate on them and press buttons. The long-tap issue is still here but it's not as bad. They're less likely to inadvertently tap stuff because they can see easier. I still have to put some stuff back into place when I visit them, but that's just normal grandparent-visiting stuff.
side note: One thing I noticed they really hated across all devices (and frankly, I hate it too) are pop-ups. Not pop-up ads because I blocked those for them, but app or system pop-ups. If they open an app and they get hit with 15 unskippable tutorial pop-ups, they will just hit the lock button and walk away. They will come back later, wake the device up, see they still have to go through with the tutorial, and then call me and ask how it "broke". I highly suggest configuring devices to minimize this as much as possible. It makes them feel like they are losing control over the device and makes them panic or get frustrated, and honestly I can't blame them. It's horrible ux design.