We aimed to examine the effects of contextual factors (ie, observers' training background and priming texts) on decoding facial pain expressions of younger and older adults. A total of 165 participants (82 nursing students and 83 nonhealth professionals) were randomly assigned to one of 3 priming conditions: (1) information about the possibility of secondary gain (misuse); (2) information about the frequency and undertreatment of pain in the older adult (undertreatment); or (3) neutral information (control). Subsequently, participants viewed 8 videos of older adults and 8 videos of younger adults undergoing a discomforting physical therapy examination. Participants rated their perception of each patient's pain intensity, unpleasantness, and condition severity. They also rated their willingness to help, sympathy level, patient deservingness of financial compensation, and how negatively/positively they feel towards the patient (ie, valence). Results demonstrated that observers ascribed greater levels of pain and other indicators (eg, sympathy and help) to older compared with younger patients. An interaction between observer type and patient age demonstrated that nursing students endorsed higher ratings of younger adults' pain compared with other students. In addition, observers in the undertreatment priming condition reported more positive valence towards older patients. By contrast, priming observers with the misuse text attenuated their valence ratings towards younger patients. Finally, the undertreatment prime influenced observers' pain estimates indirectly through observers' valence towards patients. In summary, results add specificity to the theoretical formulations of pain by demonstrating the influence of patient and observer characteristics, as well as informational primes, on decoding pain expressions.