OBJECTIVE: This study identified developmental patterns of handwashing habit formation during childhood and examined their associations with later COVID-19 preventive practices. METHODS: We used data from the Taiwan Birth Cohort Study, which included 11,254 adolescents with complete data on childhood handwashing behavior and age-15 COVID-19 survey items. Bias-adjusted 3-step latent class analysis was used to test study hypotheses. RESULTS: The rates of handwashing and mask-wearing during the pandemic were 63.8% and 93.8%, respectively. Five distinct patterns of handwashing habit formation were identified: early formation (14.89%), delayed formation (17.73%), gradual formation (42.98%), inconsistent formation (9.78%), and nonformation (14.62%). Compared with adolescents with an early formation pattern of handwashing habits, those with other patterns exhibited lower odds ratios (ORs) of handwashing during COVID-19; these ORs were 0.67 (95% confidence interval [CI], 0.49-0.85), 0.60 (95% CI, 0.44-0.77), 0.29 (95% CI, 0.08-0.49), and 0.21 (95% CI, 0.01-0.40) for those with delayed formation, gradual formation, inconsistent formation, and nonformation patterns, respectively. Moreover, relative to that of adolescents with the early formation pattern, mask-wearing was less common among adolescents with gradual formation, inconsistent formation, and nonformation patterns, with ORs of 0.54 (95% CI, 0.16-0.92), 0.50 (95% CI, 0.03-0.96), and 0.26 (95% CI, 0.00-0.65), respectively. CONCLUSIONS: The early formation of hygienic habits is associated with higher adherence to pandemic preventive practices among adolescents. Our findings suggest that interventions to promote hygienic behaviors can start as early as age 3 through the introduction of healthy habits such as handwashing.