Age differences in the prevalence of loneliness have been a key focus among researchers, practitioners, and policy makers. However, the degree to which those reflect genuine differences in the experience of loneliness or the way individuals understand and respond to loneliness measures is yet to be examined. The current study explored the age measurement invariance of the 20-item Revised University of California Los Angeles, Loneliness Scale (UCLA-LSR) and its shorter forms in a U.K. sample of adults aged 18 to 99 years ( M = 50.6, SD = 19.7). The fit of different structures/versions was explored through multigroup confirmatory factor analysis (CFA; N = 4,375) and local structural equation modeling ( N = 19,521). Results indicated a poor and/or inconsistent structure for the 20-item UCLA-LSR and many of its shorter forms. Of the structures considered, 12 showed acceptable model fit and received age measurement invariance testing through multigroup CFA and alignment; 10 of these achieved full, partial, or approximate measurement invariance. Our findings suggest that the age measurement invariance of loneliness measures should not be assumed, and crucially, this must be explored before accurate and meaningful age comparisons can be made. Implications for measurement research, and clinical and community practice, are discussed.