I'm just chiming in that I have a dd with an early fall bd who was also the youngest in her grade (or close to the youngest). We still agreed to skip her a grade and it was absolutely the right choice for her despite her age.
We've never found in class differentiation to be adequate except for one year where my oldest dd had a really wonderful teacher who ran the classroom rather differently with all kinds of centers with different types of work and a lot of parent volunteers who came in and helped at the various centers. Kids were only assigned to some of the centers (no one did everything) and the assignments as to where each kid went were tailored according to the academic needs.
