Optical spectroscopy is used to confirm membership for eight low-mass candidates in the young Beta Pic moving group (BPMG) via their radial velocities, chromospheric activity and kinematic parallaxes. We searched for the presence of the Li i 6708 Å resonance feature and combined the results with literature measurements of other BPMG members to find the age-dependent lithium depletion boundary (LDB) - the luminosity at which Li remains unburned in a coeval group. The LDB age of the BPMG is 21 ± 4 Myr and insensitive to the choice of low-mass evolutionary models. This age is more precise, likely to be more accurate, and much older than that commonly assumed for the BPMG. As a result, substellar and planetary companions of BPMG members will be more massive than previously thought.