15, 17, 18, and at age 19+, with the modal age of initiation (16 years) used as the referent group. For all survival analyses, the Efron approximation (Efron, 1977) was used for survival ties. The Grambsch and Therneau test of Schoenfeld residuals (Grambsch & Therneau 1994) was employed to examine potential violation of the proportional hazards assumption, with age-interactions modeled to correct observed violations (Cleves et al., 2004); tests equating hazard ratios across risk periods were conducted post-hoc using a p-value of 0.05 for statistical significance.
