The way I understand it is that classroom teachers expect to be teaching a range of students and are trained to do so. So a 5th grade teacher can expect to be teaching students from 4th grade level to a 6th grade level. So if your 5th grader is at a 6th grade level, the teacher should be able to handle that with possible gifted pull our and/or differenciation. YMMV I am sure, but that is supposed to be the standard.
If your child is two years or more ahead of his or her age peers in all areas, then whole grade acceleration is something to explore. If it is just in one subject area, look into subject acceleration.
Pace of teaching is going to be an issue still with acceleration, but also a younger child might be expected not to have as developed executive functions, so it may come out in the wash in that case.
Good luck! Getting a ton of information is going to help with your decisions.
