Many studies reported heart rate changes were associated with mental stress. Recently, a Stress Response Inventory (SRI) questionnaire has been devised to score physical, mental, and emotional symptoms related to mental stress occurred during the past two weeks. However, SRI has too many items to be asked routinely in a mobile device such as a cellular phone. Furthermore, its individual scores may not contribute equally to estimating stress levels in different age groups. Therefore, we sought to identify mental stress factors in the SRI that were associated with heart rate changes in different age groups. Subjects aged from 20 to 69 (n=369) scored a simplified SRI and then heart rates were measured. Multiple linear regression analysis was performed to detect subsets of SRI items that showed significant relationships with heart rates (p-value<0.1). We expect that these age dependent models could be used in identifying the cause of heart rate changes in a mobile application.