We have obtained intermediate-resolution optical spectroscopy of 44 candidate very low mass members of the nearby young open cluster IC 2391 .
Of these , 26 spectra are totally new , 14 were already analyzed in a previous paper and another four are in common .
These spectra , taken at the Cerro Tololo 4-meter and Magellan I and II telescopes , allow us to confirm 33 of them as likely cluster members , based on their spectral types , presence of Li , and H \alpha emission .
Among these new cluster members is CTIO-160 ( M7 ) , the first IC 2391 candidate to satisfy all criteria for being a substellar member of the cluster , including detection of the Li 6708A doublet .
With the enlarged membership , we are able to locate the lithium depletion boundary of the cluster more reliably than in the past .
Based on comparison to several theoretical models , we derive an age of 50 \pm 5 Myr for IC 2391 .
We also estimate new ages for the Alpha Per and Pleiades clusters ; our ages are 85 \pm 10 Myr and 130 \pm 20 Myr , respectively .
We derive an estimate of the initial mass function of IC 2391 that extends to below the substellar limit , and compare it to those of other well-studied young open clusters .
The index of the power law mass function for IC 2391 is \alpha =0.96 \pm 0.12 , valid in the range 0.5 to 0.072 M _ { \odot } .