It's very hard to define requires as well, as different personalities will handle lack of challenge in different ways. My son absorbs all that is taught, extends to some degree, and then quietly clocks out if he's not interested.
I'd agree that personality plays a lot into how the child behaves and what s/he needs. My oldest didn't handle repetition or boredom well at all when she was younger. She responded by working slower, crying, becoming anxious and depressed to the point that we were worried enough to homeschool her for a while.
I couldn't take a six year old telling me she wished she had never been born. It scared me. I think that her unusual direction and drive makes her stand out even more than her ability scores would predict. She was getting somewhat lazy in 4th grade and procrastinating terribly but her work ethic picked back up post-skip. I'm seeing quite a bit of procrastination again in her "easy" subjects but overall she's a motivated kid who is not only willing to work hard, she wants the challenge.
My little one was sort of an early admit although not technically. We just played the cards well to get her in when we did. Our state allows each district to set its own K cut-off although no one has a cut off later than Oct. 1 b/c the state only funds kids in K who are five by 10/1. The three local districts have cut-offs of June 15, Sept. 15, and October 1. The Oct. 1 district was also a Sept. 15 one up until the year dd10 started. They moved their cut-off out that year so we enrolled her there to get her in and then moved her to the 9/15 district (where she didn't make the cut-off but they let her start 1st the next year b/c she had already completed public K elsewhere). She got in at the 10/1 district by mere days. It is very common here to hold out kids whose bds fall in the summer so she's btwn a few months to 18 months younger than her grade peers.
For dd10, wherever she falls in terms of giftedness, she has a hard time with being the youngest and by far the smallest kid in 5th grade. She is also one who tunes out and daydreams if the work isn't engaging. She is never a problem for the school, doesn't fall apart outwardly, and doesn't seek challenge herself if it isn't presented. Her personality isn't conducive to further acceleration beyond the subject acceleration she is doing in math. Add the ADD & anxiety issues in, and I'm thinking her current placement is about all I'd ask for.
