I started school early. It worked out okay at first, because I was bright enough to keep up, but it caught up with me around 8th grade and I had to be homeschooled so I didn't go crazy. I was always top of my class and in the gifted program, and eventually I was just too far behind developmentally to keep up at the level I was accustomed to.
I think 6 or 7 is good... it really depends on the child and how they do with "informal" education at home. I like multi-age classrooms, like Montessori. They bridge this gap well, imo.