My nephew is 6, but he loves to read on my Kindle. We searched the age appropriate areas on Amazon for Chapter books for him, but all of the samples so far only contain the publishing info (idjits). I won't buy what I can't "sample" when it comes to ebooks, so... so far, no reader for him!
He Likes that he can make the font bigger for his books... it seems to help with letter definition (hmm... wonder if maybe he needs glasses
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Umm, so to actually Answer the question, I'd say around 6 with supervision and If we could find books for him to read!