It's the only OS for my 93 year old mother. I can manage it remotely, too, and she can't mess it up.

My mother (80+) runs Fedora, and I believe she is incapable of messing it up, even if she did have the root password. Doubleclicking random exe files off the internet is almost uniquely a Windows problem. I dunno about Macs - its users are usually technically illiterate, but Apple has done a pretty good job of locking users out of their own machines.