Age at menarche is critical in research and clinical settings, yet there is a dearth of studies examining its reliability in adolescents. We examined age at menarche during adolescence, specifically, (a) average method reliability across 3 years, (b) test-retest reliability between time points and methods, (c) intraindividual variability of reports, and (d) whether intraindividual variability differed by setting or individual characteristics. Girls (n = 253) were enrolled in a cross-sequential study in age cohorts (11, 13, 15, and 17 years). Age at menarche was assessed using 3 annual, in-person clinician interviews followed by 9 quarterly phone interviews conducted by research assistants. Reliability of age at menarche across time was moderate and varied by method. In-person interviews showed greater reliability (intraclass correlation coefficient [ICC] = 0.77) versus phone interviews (ICC = 0.64). Test-retest reliability in reports did not decrease across time. However, average differences in reported age varied as much as 2.3 years (SD = 2.2 years), with approximately 9% demonstrating differences greater than 4.5 years. Pubertal timing category (i.e., early, late) changed for 22.7% if categorized at the final versus the first report of age at menarche. Reliability was moderate, but average differences in reported age were notable and concerning. Using in-person clinician interviews may enhance reliability. Researchers and clinicians should be cognizant of the implications of using different methods measuring age at menarche when interpreting research findings. (Contains 4 tables.)