So I've heard a couple of times about writers for The Economists being college interns and in a recent 
ep (195) the guest referred to them all being 19. I was wondering if there's any actual documentation on that, or is it just vibes based?
That's just vibes based. You can browse their writers and editors here, they're actual journalists for the most part: https://mediadirectory.economist.com/
The Economist every year also runs a "Books by our Current Writers" section in their last issue of the year, it's definitely not mostly interns/college kids. Their articles are anonymous so the magazine "speaks with one voice" (the voice of British millionaires!) but the people they employ are all known. Their columnists, they get their own article in every issue, are also not anonymous.