counter anecdotes: my 73 year old grandma switched to Linux mint 9 months ago and loves it. and my 21 year old cousin tried the same and ran back to Windows after days.
The issue is, for a person as old as your grandmother, learning even the barebones of how to use a computer can be a novel experience because they have had a very small chunk of their entire life to attempt it. This is following having access to many other modern forms of entertainment for far longer.
Whereas, in the case of the young cousin, they grew up with tech like this and having to relearn something that is basically the same system but with a different coat of paint and more inconveniences is a hassle.
I'm not much of a Linux user (I only check out some distros once in a while), but I found Linux Mint Cinnamon to be extremely intuitive, and I "got it" right away. I actually thought it was easier to use than Windows 7, and I use Windows 7 all the time.
I also think Zorin OS is looking great, and it seems like a very nice OS for those moving from Windows 7, especially.
