2026AJ....171..143S

The Ursa Major Moving Group (UMa) is one of the closest stellar associations, yet its age has remained controversial, with published estimates ranging from 200 Myr to 1 Gyr. We present a comprehensive age analysis using the largest sample of candidate UMa members to date. Using Gaia DR3, we identify 1172 stars within 100 pc of the Sun with 3D kinematic motions consistent with group membership. We determine the age of UMa's dominant population using three independent methods: lithium equivalent widths ($393.{6}_{-80.9}^{+85.1}$ Myr), gyrochronology (428 ± 93 Myr), and photometric variability indicators ($44{9}_{-79}^{+114}$ Myr). The three methods converge on a consistent age of $41{8}_{-34}^{+32}$ Myr. While our kinematic selection includes field stars that share UMa's space motion but are not coeval members, the convergent age determinations clearly identify a dominant population that formed together 400 Myr ago. These stars are important benchmarks for studies of stellar rotation, magnetic activity evolution, and lithium depletion. The presence of systems such as HD 63433, a young multiplanet host within the group, further illustrates the value of UMa as a laboratory for early planetary system evolution. Our expanded catalog of kinematic candidates lays the groundwork for spectroscopic membership confirmation, refined mapping of the group's structure and chemistry, and future investigations of both stellar and planetary evolution at this key epoch.