Context : Young populations at Z < Z _ { \odot } are being examined to understand the role of metallicity in the first phases of stellar evolution .
For the analysis it is necessary to assign mass and age to Pre–Main Sequence ( PMS ) stars .
While it is well known that the mass and age determination of PMS stars is strongly affected by the convection treatment , extending any calibration to metallicities different from solar one is very artificial , in the absence of any calibrators for the convective parameters .
For solar abundance , Mixing Lenght Theory models have been calibrated by using the results of 2D radiative-hydrodynamical models ( MLT- \alpha ^ { 2 D } ) , that result to be very similar to those computed with non-grey ATLAS9 atmosphere boundary condition and full spectrum of turbolence ( FST ) convection model both in the atmosphere and in the interior ( NEMO–FST models ) .

Aims : While MLT- \alpha ^ { 2 D } models are not available for lower metallicities , we extend to lower Z the NEMO–FST models , in the educated guess that in such a way we are simulating also at smaller Z the results of MLT- \alpha ^ { 2 D } models .

Methods : We use standard stellar computation techniques , in which the atmospheric boundary conditions are derived making use of model atmosphere grids .
This allows to take into account the non greyness of the atmosphere , but adds a new parameter to the stellar structure uncertainty , namely the efficiency of convection in the atmospheric structure , if convection is computed in the atmospheric grid by a model different from the model adopted for the interior integration .

Results : We present PMS models for low mass stars from 0.1 to 1.5 M _ { \odot } for metallicities [ Fe/H ] = -0.5 , -1.0 and -2.0 .
The calculations include the most recent interior physics and the latest generation of non-grey atmosphere models .
At fixed luminosity more metal poor isochrones are hotter than solar ones by \Delta \log T _ { eff } / \Delta \log Z \sim 0.03-0.05 in the range in Z from 0.02 to 0.0002 and for ages from 10 ^ { 5 } to 10 ^ { 7 } yr .

Conclusions :