A time-processing deficit has been proposed as a neuropsychological candidate endophenotype for Attention Deficit Hyperactivity Disorder (ADHD), but its developmental trajectory still needs to be explored. In the present study, children (N = 33) and adults (N = 22) with ADHD were compared to normal controls on two time-processing tasks. For time reproduction, ADHD-related impairment was found in the full group, but not when adults were analyzed separately. For the discrimination of brief intervals, children and adults with ADHD showed different patterns of deficit. We conclude that in ADHD some time-processing deficits are still present in adults, but may take on age-related different forms.