Every child is different, and I am a big believer in parents being the most qualified to assess what will be the best, or most appropriate environment for their child. I also know it isn't always an easy assessment to make, and other people's experiences can be informative, so for what it's worth, we started our DD in K at 3.8 (she has a January birthday and wouldn't have been traditionally eligible for another 2 years).
For her, it was definitely the right thing to do. She had started preschool at 2.8 (at the encouragement of her toddler care provider), and her preschool director was the one who urged us to have her tested at 3. According to her preschool director, she had already mastered all of the multi-year curriculum there, and she felt strongly DD was ready for K. The psychologist who tested her agreed.
We considered giving her one more year of preschool, but given where she was and her pace of learning, we felt fairly certain she would be past a K curriculum after another year. A jump from preschool to K seemed like a better transition then a jump from preschool to 1st grade. Most importantly, we talked to DD and told her we, and her teachers, felt she was ready for K but it was her choice whether to go to preschool or K. She wanted to go to Kindergarten, so off we went. I can't say it was smooth sailing. It was for DD - she loved it and thrived. It was a good fit for her academically and socially. It was rough for us because many of the other parents in the class were absolutely awful about it initially. The teachers were super supportive, but they did struggle some, not having a lot of training or experience in gifted education. At some points they seemed to expect her to achieve perfection in everything. Ultimately the school brought in a consultant for them and that was incredibly helpful.
She moved on to first grade this year when she was 4.8, and this year has been even better. She is comfortable and confident, and has made great progress without being frustrated. Now we're faced with what do do next year, however, because another acceleration at some point seems likely. That's a whole different issue, though.
Trust your gut. If you feel like she's ready, she probably is. Also, if you don't feel like you're seeing what her scores indicate, it could be because she isn't being asked to perform to that level. Our DD's teachers have learned that often they just need to put something in front of her even if they aren't certain she's ready, and most of the time she rises to the challenge without any trouble. Finally, I do think early K entrance is a good transition point. If you think you'll need to accelerate her, this is a great time to do it.
