Why? Well can your characters read and write? The answer to that is not necessarily. Your society could have levels of illiteracy. You could be looking at why that is and whether anyone is trying to change it. Equally your hero/heroine may come from an educational background not usually known for heroics, so making their story more remarkable, but there has to be some feel for what is considered a normal education in your societies so your readers can pick up on that point.
How does the education your characters have had help them (or otherwise) with the lives they are living now? Are they making up for lost time by increasing their education now? What got in their way that stopped from doing this at the "normal" age?
Interesting points to ponder I think - and there could be some cracking stories in these thoughts.