children ages 7-10 had a harder time standing still when compared to older children (12-15) or adults due to immaturity in their sensory readjustment systems (proprioceptive weighting).
The visual system also contributes significantly to postural balance, so don't be surprised if kids with visual problems have problem standing (or sitting) still too.
Hmm. Makes you think about what we're expecting of young kids and when educational or behavioral expectations will catch up to developmental science.
Photo : rocket ship from Flickr