I don't know who those people are, but a large number of my almost-adult students don't know how to attach documents to email, use the basic tools of a word processing program, or insert an image into a document. it's because, while they grew up with smart phones (and they all have twitter), they DIDN'T grow up with computers and most of them still never use a computer anywhere other than at school.
so that's one place computer illiterate kids might come from. poverty & underprivilege, and just a different set of societal priorities & expectations.
These are definitely the sorts of issues we're getting into in my LIS program in re: the digital divide, computer and tech literacy, community and economic development and empowerment, etc. It's a really complicated picture with lots aspects you wouldn't expect, and our desktop-based computing skills (and even moreso, our handcoding, self-hosting, DIY web skills) and our notions about culture around these tools and literacies now look very time, place, and class bound to me.
They also happen to be dominant in the more stable and white-collar parts of the professional world, which is pretty crappy for a lot of people who have really rich technological and cultural literacies and skills, but who either missed or skipped the desktop computing moment and went straight to mobile access and social media.
Lots of kids (some from underprivileged backgrounds, some not, though it's easier on the more privileged kids and they have less of an excuse) are definitely having to seriously adjust their approaches to media and technology once they hit college and the professional world, and educational systems and adults aren't giving them much practical guidance at all when it comes to that.