Excerpt from this article:
You saw it. We saw it. Everyone saw it. Slowly but surely this past weekend everyone started posting current pictures of themselves next to photos from 2009. Largely marked #2009vs2019 or #10yearchallenge the posts flooded Twitter, Facebook, and Instagram. As might be expected, the meme quickly got picked up by those looking to make a point about the state of the world—or get LoLs—through posting political images or jokes about celebs, but the more personal side of it, the actual nostalgia bit, is a gift given to us by our devices.
The reasoning is simple: By 2009, thanks to the boom in smartphones, most folks had very decent cameras in their pockets at all times.