No advice because it has to be right for what is your child. But here is my experience...
We held back, and there are issues both ways.
Ours has DCD and was recently diagnosed with disorder of written expression. I have been posting about this on another thread. If you are reading around, you will see it. When she as 4 and in pre-k, she developed severe anxiety issues and because her bday was a few weeks before the cutoff, drs and preschool teachers encouraged us to give her the gift of time. So we gave her the gift of time for motor and writing and held her back intellectually. I still think we did the right thing, but it is still a really uncomfortable, frustrating fit. Our situation is different than yours because she started k right at 6, so she is older than grade, not younger.
Disadvantages: 1) any time you aren't making the school approved age/grade decision, when there are problems they can just say, "well if x was in the right grade, this would be different" and you can't prove them wrong, 2) mine is ashamed to be in her grade and would love to be a grade higher, 3) mine would have more academic peers in the next grade up, 4) other parents don't see my child's weaknesses because they aren't verbal and I know some of them think we are trying to "game the system," 5) she does get bored in school and her mind does wander and 6) I wonder if teachers would have been more gentle about her undiagnosed LD if she didn't seem so cognitively strong compared to the younger kids in the classroom.
Advantages: we DO need time to figure out how to cope with the LDs that have caused her so many problems, 2) because she is old for her grade it is easier to not have to live with the "well she is young for her grade, let's wait and see," 3) because she's not struggling to learn academics in school, we can save the frustration solely for the LDs.
She is a self motivated learner, so holding her back academically doesn't keep her from learning. In fact, she has more time to follow her interests. Our state allow them to place out of a grade if the make 90s or above on a series of grade level tests. The trick is that some of those tests are content specific and want answers that aren't the best logical answers and need to be parroted back from the curriculum. But once we are confident that she can deal w the LDs, I'd have no problem with lettin her read the next grade up textbooks and taking those tests, if she wants to.
