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The Distribution of Users’ Computer Skills: Worse Than You Think (2016)
(www.nngroup.com)
This is a most excellent place for technology news and articles.
I'm a 20-30 something (26) and, generally, I think it's younger people who are starting to struggle with this (<21 maybe?). All of my classmates seemed to be OK at handling files. From what I saw in highschool and college it was more to do with computer hygiene than incapability.
I am 22 and yeah I have some friends and some younger relatives that don't know how to do that. It's because they grew up with smartphones. I think we were one of the last generations (at least where I live) that smartphones only became prevalent after we were teenagers so using a computer for most things was still something we learned.