Early but not totally implausible. My mother taught me to read when I was four and I could write before I was five. It was a pain at school where I had to endure a couple of years of reading simple John and Jane books while everyone else was catching up. Unfortunately, while I could write, I never developed the technical skill of writing legibly and fast so even I have difficulty in deciphering quite a lot of my writing. More than thirty five years of using computers to create documents hasn't improved things either.