I maintain my parents computers for some years already. They are in their mid 60s. They are using Ubuntu with Gnome (2) for about 5 years.
There were some road blocking bugs along the years but they were all fixed (as far as we are concerned). All I can say is that now "it mostly works". Like all normal people, they need less bugs, less suprises and better/safer applications.
Now that I see Unity coming from one side and Gnome3 from the other, with all their brand new usability bugs ("oh install a plugin/extension to fix that..."), I did the only sane thing I could: bought them 1 Ipad. I believe that they will try it, learn it and use it more than either the desktop or laptop. If there is need, I'll buy another iPad so that each of my parents has one.
IOS has problems but I believe that there is no alternative today that matches its level of end-to-end support and quality.