Autism spectrum disorder is 90% genetic but responds dramatically to intensive early training. Recent reports indicate many of the genetic variations associated with ASD involve activity-dependent regulation in the brain, or synapse development that depends on postnatal learning and experience. Intensive training can apparently overcome the effects of these genetic variations, but this should be started as early as 12 months of age for best results. The proposed reason is the decline in brain plasticity after about 6 years. In addition, the problems caused by ASD may cascade on a trajectory toward full expression of the disorder, making it important to interrupt this sequence before it builds. The possibility of reopening plasticity to increase effectiveness of intervention later in life is discussed.