I've taught a basic web programming class to 17/18 year old's. Hardly anyone had ever heard of file extensions (windows by default doesn't show them anymore), and most of them didn't understand the concept of folders and files, at all. I was shocked.

I spent 4 hours with them before the whole class was able to create a "index.html" file inside a specific folder, it was like teaching old people. I now feel a lot safer in my programming job.