Dietary methionine restriction (MR) improves biomarkers of metabolic health, in part through coordinated increases in energy intake and energy expenditure (EE). Some metabolic benefits of dietary MR are secondary to its effects on energy balance, so this study's purpose was to examine how age at initiation of MR influences its effects on energy balance and body composition. Energy balance was examined in rats provided control or MR diets for 9 months after weaning or in rats between 6 and 12 months of age. Rats provided the control diet for 9 months after weaning increased their body weight (BW) and fat mass by five- and eightfold, respectively, while BW and fat accumulation in the MR group were reduced to 50% of that of controls. In adult rats fed the respective diets between 6 and 12 months of age, dietary MR increased energy intake by ∼23%, but the 15% increase in EE was sufficient to prevent increases in BW or fat mass. Dietary MR produces comparable increases in EE in young, growing animals and in mature animals, but young animals continue to deposit new tissue because of the proportionately larger effect of MR on energy intake relative to maintenance requirements.