A year ago, we were in a very similar situation as you are. DS has a March birthday (so, young for his cohort). But he was reading by 3 years 5 months, and by 4 years, was fluently reading K-1 level books. We tried to work with his preschool to have them engage him cognitively in some way during the day, but although they meant well, it was clear they didn't really have experience with HG kids (they started by emphasizing handwriting, which was absolutely the wrong thing to do).
We live in NC, which has a statewide policy about early admission to K, so we followed their procedures which included paying for private testing and meeting with the school. Testing was expensive, but it was very helpful to have hard numbers showing DS at 99.8/99.9 across the board.
Our local elementary let him start K early, and I have to say it's been a mixed bag. Because he is so young, he has struggled with some things, including being overwhelmed by the noise/activity level of so many kids, dealing with a teacher who does not at all "get" giftedness and who is still making the poor boy match lowercase and capital letters, and classroom peers who are sometimes quite a bit older (1.5 or more years).
Has there been an academic benefit? I don't know. His reading hasn't really improved. His math abilities have improved, but his love of learning is just about zilch when he's at school. (Not so at home.)
On the other hand, he really didn't like preschool, either, so early K felt to me like the least worst option. Overall, I'm not sure I'm happy that we did it, but it also hasn't been disastrous.
I think a lot of my reservation has to do with the school itself; it's a Title 1 school with a high ESL population, so they are dedicating a lot of resources towards helping the students meet state standards. It doesn't seem that they have much time/energy left for figuring out how to engage the gifted kids. YMMV depending on your school.
I will say that the disconnect between his emotional maturity and his cognitive needs has been a really sticky problem for us. We're considering enrolling him in a private school for gifted students next year because we don't have any faith that the public school system is going to be able to meet him where he is.
In short: what Dude and cmguy said. Early K works for some kids, not for others. My kid's an introvert, which I think has worked against him. He doesn't really have any close friends at school. The school psychologist swears that he fits right in and no one can tell that he's young for his grade, but we still worry about the long-term effects of feeling disconnected from both peers and the subjects they're studying.
