The Early Start Denver Model (ESDM) is an intervention program recommended for pre-schoolers with autism ages 12-48 months. The rationale for this recommendation is the potential for intervention to affect developmental trajectories during early sensitive periods. We investigated outcomes of 32 children aged 18-48 months and 28 children aged 48-62 months receiving the ESDM for one year (approximately 20 h per week). Younger children achieved superior verbal DQ gains compared to their older counterparts. There were no group differences with respect to non-verbal DQ and adaptive behavior (with both age-groups undergoing significant change), or ASD severity (with neither age-group showing improvements on the ADOS). The association between verbal DQ gains and age at intake was moderated by baseline verbal level.