The early onset of alcohol and tobacco use has been associated with increas
ed risk for later substance abuse and dependence problems. This study inves
tigated genetic and environmental influences on age at onset of alcohol and
tobacco use by examining twin resemblance for several retrospectively repo
rted onset milestones including age at first use, age at first alcohol into
xication experience, and age at regular use. In addition, we also examined
the latency between age at first use and age at regular use of tobacco and
alcohol. The subjects were a volunteer sample of older adult twins 50 to 96
years of age. MZ twin correlations for age at first alcohol use and age at
first tobacco use were .57 and .44, respectively, compared to .45 and .37
for DZ same-sex twins. MZ twins correlated .30 and .26 for the latencies be
tween first use and regular use of alcohol and of tobacco, while DZ correla
tions were -.01 and .05, respectively. Biometrical model-fitting results co
nfirmed that familial resemblance for age at first use for both alcohol and
tobacco was largely the result of shared environmental factors, while the
latencies between first use and regular patterns of use were more genetical
ly influenced. These findings add to a growing body of literature suggestin
g that initiation of substance use is influenced primarily by environmental
rather than genetic factors.