This study aimed to develop and validate an instrument for the assessment of patient-relevant benefit in dermatocosmetic treatment, i.e., skin care, of aged skin. Based on an open item collection with 33 elderly persons, items on patient-relevant treatment goals were collected. An expert panel selected 20 items to be most relevant and feasible for the questionnaire named Patient Benefit Index for Aged Skin (PBI-AS). The instrument, which assesses goal importance and achievement, was tested in a cognitive debriefing and validated in a longitudinal study (n = 80) along with the Dry Skin Area and Severity Index (DASI) and the Dermatology Quality of Life Index (DLQI) as convergent validation criteria. The cognitive debriefing showed the good practicability and feasibility of the instrument. Significant correlation with change in DASI (r = -0.527; p < 0.001) supports convergent validity of the PBI-AS. By contrast, correlation with DLQI was poor, indicating the different constructs. The PBI-AS is a valid and feasible tool for the patient-centered assessment of dermatocosmetic treatment benefit in aged skin.