I'd agree with AlexsMom that there are a lot of kids whose parents or others say that they are reading at a 5th grade level in 1st grade but that it doesn't necessarily mean the same thing it means in a child who actually comfortably reads at that level. My kids are girls and never read that particular series, so I don't know what exactly it looks like.

I do know, however, that my oldest was reading Harry Potter books pretty independently by 6.5 and that she was probably the most advanced reader in her 2nd grade class, which she started shortly before her 7th bd, so just a bit after that. There were definitely a number of other kids who were advanced readers, but more typical at 6-7 were the advanced readers who could read books like Junie B. Jones.