The earlier, the better. Plenty of clinical research has demonstrated that children who start ABA therapy as early as possible (as young as 18 months old) make the most significant improvements in their areas of deficit. In fact, children who received early intensive ABA therapy progressed to the point of being virtually indistinguishable from their neurotypical peers. A child that begins therapy young (2-5 years old) while still in the traditional “brain development” stage will have a higher likelihood of achieving full developmental capability than a child that starts therapy at a much later age. There’s no strict rule of thumb on age other than starting early intervention as soon as possible.