My daughter is 4 and has already been a flower girl 3 times and each time walked down the isle with huge smile. I think 6 is a great age! It all depends on the child. If they are a social child, they will be fine.
I have 2, my 7 year old neice who is super excited has been asking to be one years before we were engaged,. she can't wait to throw petals. Also FH god-daughter who is 2, we don't expect her to do anything but walk down the aisle.