When I was in 4th-5th grade I thought the Macarena was of similar vintage to songs like the YMCA and the Electric Slide, because they were commonly played along side each other at parties and such the time. Had no idea it was in fact brand new.
I was in college by the time I realized "Run Around" by Blues Traveler and "Good Riddance (Time of Your Life)" by Green Day were from the 1990s and not the 1960s.
Meanwhile when I first heard "Crazy" by Gnarls Barkley (in a video of a cat on a treadmill) I assumed it was some old song from the 70s that I had managed to never hear until then. Nope, it was new.
For an example of the opposite, I was surprised recently to learn that "(Don't Fear) The Reaper" by Blue Öyster Cult came out in 1976. I had had that pegged as such an 80s song.