I get that this is a ridiculous nit-pick to kick off a comment, but how do they define "young adult?" Because I'm not sure that that statement is actually true just yet.
I have two Gen Z daughters, the youngest having been born in 2001. Smart phones didn't really start to land on the scene until 2008. It then took a few years until everyone had one and the cameras were good enough that people were getting rid of their digital cameras and switching to smart phones.
Granted, my daughters are several years older now than an 18 year-old, which I would consider to be the "youngest" possible "young adult."
Even then, an 18 year-old today was born in 2006. I suppose that part of their childhood was likely captured on smart phones rather than digital cameras. But being old enough that I remember the 00s like it was just yesterday, and having two "young adult" children whose childhoods' were almost entirely documented using digital cameras, even having packs of old printed photos that were printed at the same places you would get film developed ... I just can't help but question the statement that today's young adults had their childhoods documented on smartphones. Today's children and young teenagers, sure. But the modern smartphone, with camera, actually hasn't been with us for a full generation quite just yet... as much as it feels like it.