• developmental delay;
  • sensitive period;
  • specific diagnosis

This article presents current neurobiological concepts that highlight the critical role of chronological age in determining optimal development. The role of sensitive periods, experience expectancy, gene expression, and gene–age interactions is discussed. The debate between “splitters” and “lumpers” is presented in light of the review articles in this special issue. The conclusion from this study is that in a significant proportion of cases, earlier diagnoses are possible, avoiding the all-encompassing developmental delay/global developmental delay, and opening up possibilities of early interventions. It is further argued that research methodology might benefit from early diagnoses as well. © 2013 Wiley Periodicals, Inc. Dev Disabil Res Rev 2011;17:180–184.