The aims of this study were: to compare energy expenditure (EE) estimated from the existing GT3X accelerometer equations and EE measured with indirect calorimetry; to define new equations for EE estimation with the GT3X in youth, adults and older people; and to define GT3X vector magnitude (VM) cut points allowing to classify PA intensity in the aforementioned age-groups. The study comprised 31 youth, 31 adults and 35 older people. Participants wore the GT3X (setup: 1-s epoch) over their right hip during 6 conditions of 10-min duration each: resting, treadmill walking/running at 3,5,7, and 9kmh(-1), and repeated sit-stands (30timesmin(-1)). The GT3X proved to be a good tool to predict EE in youth and adults (able to discriminate between the aforementioned conditions), but not in the elderly. We defined the following equations: for all age-groups combined, EE (METs)=2.7406+0.00056VM activity counts (countsmin(-1))-0.008542age (years)-0.01380 body mass (kg); for youth, METs=1.546618+0.000658VM activity counts (countsmin(-1)); for adults, METs=2.8323+0.00054VM activity counts (countsmin(-1))-0.059123body mass (kg)+1.4410gender (women=1, men=2); and for the elderly, METs=2.5878+0.00047VM activity counts (countsmin(-1))-0.6453gender (women=1, men=2). Activity counts derived from the VM yielded a more accurate EE estimation than those derived from the Y-axis. The GT3X represents a step forward in triaxial technology estimating EE. However, age-specific equations must be used to ensure the correct use of this device.