Objectives Emerging adulthood, a potential critical period, is an understudied period of fat mass accrual. The aim of this study was to describe patterns of fat mass accrual, and weight status, from adolescence, through emerging adulthood, into young adulthood. Methods One‐hundred‐eighteen participants (59 male) were measured repeatedly for 20 years. Annual measures of height, weight, and body composition (DXA) were taken. Calculated measures included: peak height velocity (PHV), biological age (BA; years from PHV), body mass index (BMI), and percent total body fat (%TBF). Weight status groupings (normal NW, and overweight/obese OWO) were created using age and sex specific BMI and %TBF cut‐offs. Analysis included t‐tests and logistic regression. Results BMI and %TBF increased significantly until 8 years post PHV (P < .05), plateaued for 7 years (P > .05), and then began increasing again (P < .05). At PHV, 9% of males and 14% of females were OWO rising to 65% and 32% respectively 15 years post PHV. OWO status at PHV did not predict OWO status in early adulthood (P > .05). Conclusions During emerging adulthood, the prevalence of OWO increased. Being NW at PHV was not protective against being overweight in young adulthood. Emerging adulthood appears to be a potential critical period for fat accrual and warrants further attention.