The TrackIt task was developed as a measure of selective sustained attention that is developmentally-sensitive and able to partially separate exogenous and endogenous factors affecting attention regulation. However, these predictions have only been investigated within a limited set of parameters and age range (3-5 years). This preregistered study reports a systematic effort to examine performance on TrackIt in an expanded parameter space and age range. This study largely replicated and extended prior findings: across most implementations of the task, we found a medium-to-large effect of age and a small effect of condition. We also found that distractor errors were more likely given Low Exogenous support and in younger children. Contrary to the preregistered hypothesis, younger children did not benefit more from exogenous support than older children. Overall, these results contribute to the body of evidence that selective sustained attention (1) improves with age and (2) is bolstered by exogenous support.