This was written largely as a result of the "challenging problems" thread that made the rounds a while back[1]. Not really suggesting anyone copies these projects, more of a "well actually, what problems have I tackled?"
It wasn't a hobby project, but I wrote a non-preemptive multitasker / co-routine library for Turbo Pascal under MS-DOS back in the 1990s.
I wrote a Forth in Assembler for OS/2 that somehow found an audience in Germany, Forth/2. I was told that you couldn't write programs in Assembler for OS/2, so I had to prove otherwise. 9-)
Very interesting. If I try to think back, my first hobby project must have been in 10th grade. At that point I very well nearly memorized the C# part of the MSDN, as well as CLR via C# and some other programmimg books (maybe GoF?) but haven't actually written any code that does anything useful. In 10th grade everyone in our class was doing some sort of ASP.NET project for school and a lot of the other students were having trouble with the data layer and SQL stuff, so I decided to code up an ORM (a thing I knew existed, but haven't actually used) and later a utility library for exposing REST APIs which were both used by most of my classmates. It was a fun project. Later projects included a script that tells you where to live based on your public transit needs, a wordle clone for the original GameBoy, and probably other even less useful stuff.
Wow, from this comment alone I can tell you were much further ahead and smarter than your peers in terms of your ability to understand concepts and abstract, while your peers were struggling on such simple things. I can tell you were a highly intelligent child. Bravo, very impressive!
[1] https://news.ycombinator.com/item?id=38768678