I don't remember forgetting things as soon as I learned them, when I was a kid. I grew knowledge quite rapidly. Now I can re read the same thing every week and re "learn" it.
Make sure you're getting enough sleep, and carving out enough focus time. What I've realized is that while my current situation on either of those is not much different than when I was younger, my ability to learn and get by with minimal sleep and amidst distraction has gotten worse.
- at school, it's the job of the teacher to force us to repeat until we've learned something. See Anki and other SRS systems.
- as a kid you don't read to learn, you read because it's fun or because you want to try something. See what another commenter said, you learn by practicing, not by reading alone.
- you forget all that you've forget as a child :)
Also, kids like to imagine stories. In the literature about learning, one method used to improve the retention is too create vivid images about what you want to learn, in other words, imagine stories to memorize.
Relevant xkcd https://xkcd.com/208/
Kids also rehearse stuff a _lot_ => Anki/SRS
Having said that, I like to learn about learning but I don't consider myself a particularly good learner :)
