Introduction: Smoking is a known risk indicator for depression and some of the anxiety disorders. No data are available on the role of age at smoking onset in the development of depression and anxiety disorders. We examined the association of smoking onset age on the onset age of depression and anxiety disorders. Methods: Participants of the Netherlands Study of Depression and Anxiety Disorders who developed psychopathology after starting smoking were selected (N = 1,055). The dependent variable was the time to onset of psychopathology after starting smoking, and the independent variable was age at smoking onset. Results: The time period between smoking onset and the onset of depression and/or anxiety disorders was 5 years shorter for early-onset smokers than for late-onset smokers. Moreover, a greater percentage of early-onset smokers than late-onset smokers had the first onset of psychopathology within the first 5 years after starting smoking. Age at smoking onset predicted age at psychopathology onset after controlling for the effects of gender, education, and childhood trauma. When separate analyses were done for depression and anxiety disorders, this pattern of results was found only for anxiety disorders. Conclusions: A young age at smoking onset is associated with a shorter time to first onset of an anxiety disorder. This study with psychiatric patients extends previous findings in general population samples that smoking and depression and anxiety disorders are associated. © The Author 2011. Published by Oxford University Press on behalf of the Society for Research on Nicotine and Tobacco. All rights reserved.