We assess the impact of starspots on the evolution of late-type stars during the pre-main sequence ( pre-MS ) using a modified stellar evolution code .
We find that heavily spotted models of mass 0.1-1.2M _ { \odot } are inflated by up to 10 % during the pre-MS , and up to 4 % and 9 % for fully- and partially-convective stars at the zero-age MS , consistent with measurements from active eclipsing binary systems .
Spots similarly decrease stellar luminosity and T _ { eff } , causing isochrone-derived masses to be under-estimated by up to a factor of 2 \times , and ages to be under-estimated by a factor of 2-10 \times , at 3 Myr .
Consequently , pre-MS clusters and their active stars are systematically older and more massive than often reported .
Cluster ages derived with the lithium depletion boundary technique are erroneously young by \sim 15 % and 10 % at 30 and 100 Myr respectively , if 50 % spotted stars are interpreted with un-spotted models .
Finally , lithium depletion is suppressed in spotted stars with radiative cores , leading to a fixed-temperature lithium dispersion on the MS if a range of spot properties are present on the pre-MS .
Such dispersions are large enough to explain Li abundance spreads seen in young open clusters , and imply a range of radii at fixed mass and age during the pre-MS Li burning epoch .
By extension , this implies that mass , composition , and age do not uniquely specify the HR diagram location of pre-MS stars .