A reliable age estimation for the field brown dwarf LP 944-20 is accomplished through the analysis of its kinematic properties .
The space velocities of this star strongly suggest its membership in the so-called Castor moving group .
LP 944-20 can be sensibly assumed to have the group ’ s age , which is estimated to be \sim 320 Myr , and metal content , which is found to be roughly solar .
With these new constrains and the available photometry and lithium abundance , current brown dwarf models are put to a test .
Using the IR magnitudes and the lithium diagnostics , the models are able to provide a reasonable description of the brown dwarf ’ s properties ( to within a few sigma ) but yield an age which is roughly 50 % larger than our estimate .
Possible reasons for this discrepancy are discussed .