The significance of ages of first use of cigarettes, alcohol, cannabis, and stimulant drugs were investigated in a sample of young drug users entering an intervention study in London. Age of first cigarette smoking emerges as a robust predictor of age of first cannabis use, and age of first cannabis use in turn is predictive of age of first stimulant use, among those using both drugs. In this sample, ages of first use of cigarettes, alcohol, and cannabis are not predictive of whether stimulant drugs are used. In a series of regression models that take account of the influence of other factors, age of first use is found to have no relationship to levels of ongoing consumption of cigarettes, alcohol, and cannabis.