My parents, who are well into their seniors, still use their iPads on a daily basis. They don't bring it along when they leave the house (pre-pandemic), but at home, my folks gets some decent use out of their iPads.

Their primary use case is media consumption, browsing the web, ordering stuff online, messaging, and occasionally, to play games. It's ideal for them because of the screen real estate. They do less pinching to zoom than on their phones, but still portable enough to be lug around anywhere they want to settle down around the house.

On the subject of choosing a tablet OS between Android and iOS, I'd choose iOS every time. Android for my phone, iPad as my tablet.

TL;DR: Yes, tablets are ideal mobile devices for most seniors. It beats squinting on phones or firing up the PC or laptop. iPads are also generally easier to use than an Android tablet.