Maybe that says more about your Grandma. My parents are in their mid 70s, and my dad especially is the most luddite, tech hating, non-handy, person I've ever known. But both took to iOS like a baby to a pacifier. My mom has both an iPhone 4S and iPad 3. My dad has an original iPad. Where I use to have to field Mac questions from them constantly, they never (hallelujah!) call me with iOS questions.
Yeah, probably. My granny had never touched a computer in her life, so she's bound to have trouble with some of the basics.
...like what the back button does.
On the other hand, my mom's at least semi computer literate, yet I had to walk her through using my iPad a couple of times before she started getting the hang of things.
I will say that iOS is simple enough that once someone gets the hang of it, they're pretty much set for life. But it's not pick up and play. Nowhere near. Even I've had to look up how to do a couple of things when I first got my iDevices.