temperature have a well known degeneracy between age and mass. In very young (1-10 Myr) brown dwarfs this problem is most serious for the coolest and least massive objects with spectral types $>$ M8 and masses $<$ 25 M$_{\rm{Jup}}$ (see, for example, Figure 5 of @weights09, hereafter W09), where objects with planetary masses and ages of $\sim$1 Myr are not well separated in the HRD from brown dwarfs with ages of $\sim$10 Myr and masses 3 times higher. The Lyon DUSTY isochrones [@allard01] and those of @d'antona97 agree in this. In order to derive the substellar IMF, this age-mass degeneracy must be lifted.

It has been shown that source ages can be measured using gravity sensitive alkali metal absorption lines in the optical or near-infrared (@martin96; @steele95; @gorlova03; @allers07, hereafter A07; @riddick07; @luhman07; @close07). This technique can differentiate 1 Myr objects from 5 Myr objects, and 5-10 Myr objects from field dwarfs, since very young substellar objects rapidly contract to smaller radii. The radii of evolved field brown dwarfs have little dependence on age or mass [@burrows93], while the radii of very young brown dwarfs can be up to six times the size of their final equilibrium radii [@stassun06]. In 