It is now widely accepted that with autism, the earlier the intervention the better.
Autism is a neurological condition affecting the brain. When a child is younger, the brain has much more plasticity (ability to change) and so changing or teaching new behaviour is much easier. From a behavioural point of view, newer behaviours are easier to change than ones with a long history of reinforcement.
What about older children?
We are all able to learn new behaviours over the course of our entire lives and this is no different for people with autism. ABA (Applied Behaviour Analysis) is a science that is used in many areas of life and helps people of all ages and abilities. The research shows that intensive behavioural intervention can be very effective with older children1.
1. Eikeseth, S., Smith, T., Jahr, E., & Eldevik, S. (2002). Intensive behavioral treatment at school for 4-7-year-old children with autism: A 1-year comparison controlled study. Behavior Modification, 26, 49-68
2. Dawson, G (2008). Early behavioral intervention, brain plasticity, and the prevention of autism spectrum disorder. Development and Psychopathology, 20, pp 775-803