We discussed the impact of people's age on thermal comfort of the internal environment and the use of energy. The human's perception of thermal comfort is changing during the aging process. Due to changed conditions of the internal environment to achieve the desired thermal comfort, use of energy is changed too. The use of heating energy depends on heating degree day. The study of the discussed problem is based on Slovenia in 2017. It was found that at 2 °C higher internal temperature, the heating degree day and the use of heating energy have increased for 14,2 %. The ratio of the increased use of heating energy due to the impact of people's age was 2,63 % of the total use of heating energy.