Early diagnosis increases the chances of successful intervention. The immature brains of younger children are more "plastic," which enables them to change more easily. This makes intervention more effective for them. Older children still benefit but the benefits may take longer to achieve and may be based on cultivating coping skills rather than on modifying the brain, as early intervention is believed to do.
(from Sensational Kids: Hope and Help for Children With Sensory Processing Disorder (SPD) by Lucy Jane Miller, PhD, OTR)