Upright posture is complex and maintained mainly by postural reflexes in the midbrain that utilize sensory input from the visual, proprioceptive, and vestibular systems.
When the midbrain isn't fully mature or isn't functioning well these postural reflexes are comprised. You may even see returned or retained primitive reflexes. You will also noticed the individual's posture is abnormal. Maybe they slouch, their shoulders are rolled forward, their head more anterior, their is a head tilt or rotation, the back is hunched, or they have a foot flare.
Their gate maybe off too. When this happens the brainstem is struggling or at the very least not working efficiently. This means that the higher areas of brain function like cognition cannot be functioning efficiently either. The cortex's is supported by the brainstem and can not function well if brainstem is not functioning well. The cortex can not procieve it's environment correctly if the brainstem is not efficient. If the cortex does not proceive its environment correctly it can not respond in the most appropriate manner.