Good question.
Simon & Schuster says grades sixth through eighth.
Some teachers say fifth graders can read it.
Twitter, well, adults are the majority of my readers.
I've visited high schoolers who were extremely interested in the topic, and even fourth graders who were reading Genesis.
So, it all depends on who you ask. And if you ask Evangeline the Reading Queen, you can be as young as 8. (Mom said that she tried to deter Eva from reading it, but Eva wasn't having it.) Check out Eva's video blog review of Genesis.