What are the factors to consider when choosing a operating system?

Some people would probably say "Start them on Windows" because of the "user friendly" myth. This is largely because virtually everyone buys a computer and just uses what's on it (Almost always Windows.). So when they see someone use, say, Linux, they start thinking the person is a hardcore power user.

The reality here is that "user friendly" is a myth. What might be "intuitive" to someone who started out on Windows would be utterly backwards and unpleasant to a Mac OS X user.

There are people who, while entirely new to Linux, jumped straight into ArchLinux, not at all percieved to be anything at all like Windows. They did fine largely because they were capable of following directions and learning.

Anyway. "Politically correct" answer would be "Windows." But the real answer would be "what is 'intuitive' for you, but still easy to teach." Nobody was born with the knowledge to use Windows, OS X, or Linux. It's all down to where the person starts.

I, for example, actually started on DOS when I was 3. If I could use DOS at 3, most people new to computing could handle Ubuntu or Linux Mint at the very least if that's what you're more familiar with.

Long answer short: There's no universal "new person" OS, even an OS marketted as "user friendly" and "intuitive."