Autism Spectrum Disorder (ASD) diagnostic tools are not designed for mental ages (MA) below 12 months. Autism Diagnostic Observation Schedule (ADOS) and Childhood Autism Rating Scale (CARS) were examined in 2-year-olds with ASD-low MA (n = 53), Global Developmental Delay (GDD; n = 175), and ASD-higher MA (n = 425). ADOS and CARS demonstrated similar agreement with clinical diagnosis, but ADOS over-classified ASD in low MA, whereas CARS both over- and under-classified. All ADOS items differentiated GDD from ASD. Elementary social behaviors (e.g., eye contact, social interest) were similar across ASD groups, although advanced skills (e.g., pointing, play) were more impaired in ASD-low MA. ASD-low MA, a severe presentation, may require a modified ADOS algorithm to account for developmental delays.