Aims To examine the heritability of cannabis initiation, the influence of a possible twin-specific environment and the influence of age on the effects of genes and environment in Dutch adolescents and young adults. Design Genetic structural equation modelling was used to partition the variance in the liability to cannabis initiation into genetic and environmental components. Setting All participants were registered with the Netherlands Twin Register. Participants A total of 6208 twins (age 13-20) and 1545 siblings (age 11-25) from 3503 families participated in this study. Measurements Self-reported cannabis use was assessed prospectively with the Dutch Health Behavior Questionnaire. Findings At the median age of the sample (16.5), genetic factors explained 40% of the individual differences in liability to cannabis initiation. Twins resembled each other more than non-twin siblings, which could not be attributed to the age difference between non-twin siblings. Environmental influences increased with age. This increase applied to environmental factors shared by twins (47% of the variance), environmental factors shared by twins and siblings (24%) and environmental factors unique to an individual (13%). Conclusion The heritability of the liability for cannabis initiation is higher in adolescents than in young adults due to a larger contribution of environmental factors in young adults. This is due mainly to environmental factors only shared by twins and those shared by all offspring growing up in the same family, but the contribution of environmental factors specific to individuals is also larger in young adults. © 2011 The Authors, Addiction © 2011 Society for the Study of Addiction.