J.J.
I think you should ask her opinion. If she has friends she doesn't want to leave behind, then perhaps leave her where she is. Social pressures are often more difficult than academic ones. However, if she wants the challenge then by all means move her. If she is more mature than kids her age, she will probably have an easier time making friends with slightly older kids anyhow. I had the chance to skip 4th grade and my parents didn't take it. I always wished I could have done it. Granted, she is younger, but a child still has opinions that should be considered.