At what age should I start ABA for my child with Autism?
Although, research has demonstrated early intervention for children with Autism but it's never too late to start ABA training. The strategies of Applied Behavior Analysis can help people with Autism of all ages. However, ABA as an autism treatment for children can be started as early as possible. The sooner you begin the intervention the better. It is usually recommended to start before the age of 3. To find an ABA therapist closest to you, access the Autism Connect Worldwide Directory.
Most children are between 2 and 6 years old when they begin ABA treatment. If a child starts at age 2, ABA can help them develop better communication skills and help them learn to follow directions and simple commands, to prepare them for preschool. For older children, ABA is often used as part of the child’s education, to teach social skills, daily living skills or to help change problem behaviors. An ABA therapist first assesses the child’s level of understanding and then devises a learning strategy.
The right age for a child to begin working with an ABA therapist is as soon as he or she shows signs of autism or is diagnosed with a developmental disorder. The sooner a child receives early intervention services, the better equipped he or she will be to become a productive learner in the classroom. According to a well-renowned source, in a study with toddlers, intensive behavioral intervention helped all ages, but those who started before age 2 were most likely to make dramatic gains.

References
https://www.chop.edu/health-resources/applied-behavior-analysis-aba-children-autism
Other Questions
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Who can provide ABA for my child?
Does ABA work for High Functioning children?
What does an ABA Program involve?
Can I use ABA therapy at home with my child; if so, how?
Won't there be too many demands placed on my child during ABA therapy? Should I be worried?