Essentially, a 6 year old and a 96 year old need to be able to use the system.
Actually, those two demographics are the easiest to convert. While my mom isn't 96 by a long stretch, she uses Ubuntu and has no problems whatsoever. Her computer literacy is close to 0.
The problem users are those we call "power users". People that have used Windows for years and know the ins-and-outs, but do not know them deep enough. They can pretty much be found in the 20-65 demographics, also known as those of working age. My dad falls in the power-user demographic and he still uses WinXP. That said, he is very open to Linux and understands it well enough to use it.
Do note that you said "use". The system still has to be set up by someone who knows what he does.
My 5 year old niece loves Ubuntu, while people who have used windows for a long time find it hard to appreciate or accept anything new. It is very natural, people would have personalized their computers over time, they even know what icons sit where on their desktops and they are very comfortable with it, or their focus is beyond using the computer itself, their focus is about getting something done. introduce a completely new OS and it is hard for them to cope, however good or bad the new OS may be. It is always hard to make a switch when switching is not really your priority. People dont want to switch unless there is a huge reward.
"Time is money and money can't buy you love and I love your outfit" - T.H.U.N.D.E.R. #1
