Fenske, E C, Zalenski S, Krantz P J & McClannahan L E, AIDD, Published online 10 November, 2005

The concept of critical periods for development are well known to child health professionals. This study by Fenske in 1985 was one of the first to elegantly demonstrate the difference that timely intervention can make. Children with Autism Spectrum Disorder (ASD) who were enrolled in an intensive behavioural intervention programme before 5 years of age had better outcomes than those enrolled after they were 5 years old.

http://www.sciencedirect.com/science/article/pii/S0270468485800057