We investigated whether the link between early pubertal timing and initiation of weekly alcohol use is mediated by changes in perceived parental alcohol-specific rule setting and changes in perceived proportion of drinkers in the peer group. Longitudinal data including 3 annual waves were used to estimate the hazard for adolescents to initiate drinking alcohol using Cox proportional hazard structural equation models in 1,286 Dutch adolescents (50.2% boys) 13-14 years old at baseline in 2008. Early pubertal timing increased the risk to initiate weekly alcohol use. However, this risk was entirely mediated by a large increase in the perceived proportion of drinkers in the peer group and a large decrease in the frequency of perceived alcohol-specific rules for early pubertal timers within a period of 1 year. There is no direct risk for early pubertal timers to initiate weekly drinking per se but an indirect one via changes in their social environments, that is, a large increase in the perceived proportion of drinkers in their peer group and parents becoming more lenient in their alcohol-specific rule setting. It is important to motivate parents not to relax their alcohol-specific rule setting over time, particularly parents of early pubertal timers.