This case report examined the characteristics of body composition, sleep quality, and autonomic nerve activity in active older adults with a younger body age—calculated from age trends in body composition and basal metabolic rate. We selected two cases who had a metabolic age younger than their actual age. They had good sleep quality, low sleep quality scores, no sarco-penia, strong muscle and grip strength, and balanced autonomic nervous system activity. Fur-thermore, they were determined to be much younger than their actual age. They were compared with two other age- and sex-matched cases, who had poor sleep quality, unbalanced autonomic nervous system activity, and had a physical age closer to their actual age. Older adults with more muscle mass and higher basal metabolism were younger than their actual age, had a better sleep status, and a good balance of autonomic nervous activity in the exercise stimulation. They also had lower percentages of body- and visceral fat and higher body water. Meanwhile, the older adults with standard muscle mass and basal metabolic rate had a poor sleep status and no sympathetic hyperactivity in the exercise simulation.