It has been suggested that adult metabolic dysfunction may be more severe in individuals who become obese as children compared with those who become obese later in life. To determine whether adult metabolic function differs if diet-induced weight gain occurs during the peripubertal age vs. if excess weight gain occurs after puberty, male C57Bl/6J mice were fed a low-fat (LF; 10% kcal from fat) or high-fat (HF; 60% kcal from fat) diet starting during the peripubertal period (pHF; 4 wk of age) or as adults (aHF; 12 wk of age). Both pHF and aHF mice were hyperinsulinemic and hyperglycemic, and both showed impaired glucose tolerance and insulin resistance compared with their LF-fed controls. However, despite a longer time on diet, pHF mice were relatively more insulin sensitive than aHF mice, which was associated with higher lean mass and circulating IGF-I levels. In addition, HF feeding had an overall stimulatory effect on circulating corticosterone levels; however, this rise was associated only with elevated plasma ACTH in the aHF mice. Despite the belief that adult metabolic dysfunction may be more severe in individuals who become obese as children, data generated using a diet-induced obese mouse model suggest that adult metabolic dysfunction associated with peripubertal onset of obesity is not worse than that associated with adult-onset obesity.