This question really shows how differently things work on your world. No one would ever send a child to school. That would be a complete waste. Children don’t understand how valuable education is, it would just be lost to them. Most people agree that unless someone is very dedicated at an early age, you shouldn’t go to school any earlier than your third or fourth life.
In any life when you’re going to be having significant education, it’s best to start immediately. Early in a life is when it’s easiest for your brain to absorb knowledge. I’ve been able to afford a bit of schooling beyond what’s really necessary, but not as much as I would like. It can be expensive, and the time it takes out of a life makes it very easy to just put off until later.
Also… 15% of people on your world can’t read? How in all the savage world can they even get along in life without that? How can you manage to not learn to read? It’s just something… you just pick it up accidentally, even if you aren’t trying. My parent taught me when I was very young, and it’s typical for parents to teach their children basic reading/writing, algebra, history, etc.