I have kept just about everything from 3rd to today. It's a whole pile of papers, old sketchbooks, and some other things I drew in.
I especially like keeping most of my work for the nostalgia factor; it brings me right back to where I was, how I was, and what I was thinking when I drew that doodle or art. As someone who can't remember the majority of middle school and parts of elementary, it really helps me remember which is great.
I also love keeping all of my old oc work. I often have to go back and look through those if I'm thinking about their character design, any lore I wrote down somewhere else, or for inspiration. I don't think my characters would be as fleshed out if I didn't have their original drawings to reflect upon and add to.
Wrote a lot more to this than I needed to whoops-