Objectives <p>The 15-item version of the Five Facet Mindfulness Questionnaire (FFMQ-15) has been widely used with samples of varying ages without adequate psychometric evaluation. This study was the first psychometric assessment of the FFMQ-15 with a lifespan sample, spanning youth to older adulthood.</p> Method <p>A gender-balanced sample of 1610 participants with ages ranging from 14 to 90 (<i>M</i>_age = 45.64, SD = 21.10) was recruited. Participants completed a questionnaire that included the FFMQ-15 and several convergent and divergent variables: well-being, life satisfaction, self-compassion, depression, anxiety, and stress. Confirmatory factor analyses were used to assess the FFMQ-15 factor structure and the model invariance of the FFMQ-15 across four age cohort groups (i.e., youth, 14–24&#xa0;years of age; younger adults, 25–44&#xa0;years of age; middle-aged adults, 45–64&#xa0;years of age; and older adults, 65 + years of age). The convergent and divergent validity of the FFMQ-15 was evaluated through correlations with the convergent and divergent variables.</p> Results <p>A five-factor intercorrelated model was the best fitting for the data. Convergent and divergent validity was generally upheld. Invariance testing suggested that, with the exception of one item, the FFMQ-15 items performed similarly across age cohort groups. However, the average scores on these facets differed across age; on average, youth had the lowest scores and older adults had the highest scores on the facets.</p> Conclusions <p>The findings have implications for how the FFMQ-15 should be used with older versus younger samples. Some caution should be exercised when using the FFMQ-15 with samples covering a wide age range, with particular attention to potentially elevated mindfulness levels among older adults.</p>