  We show that non-magnetic models for the evolution of pre-main-sequence (PMS)
stars *cannot* simultaneously describe the colour-magnitude diagram (CMD) and
the pattern of lithium depletion seen in the cluster of young, low-mass stars
surrounding $\gamma^2$ Velorum. The age of 7.5+/-1 Myr inferred from the CMD is
much younger than that implied by the strong Li depletion seen in the cluster
M-dwarfs and the Li depletion occurs at much redder colours than predicted. The
epoch at which a star of a given mass depletes its Li and the surface
temperature of that star are both dependent on its radius. We demonstrate that
if the low-mass stars have radii ~10 per cent larger at a given mass and age,
then both the CMD and Li depletion pattern of the Gamma Vel cluster are
explained at a common age of 18-21 Myr. This radius inflation could be produced
by some combination of magnetic suppression of convection and extensive cool
starspots. Models that incorporate radius inflation suggest that PMS stars
similar to those in the Gamma Vel cluster, in the range 0.2<M/Msun<0.7, are at
least a factor of two older and ~7 per cent cooler than previously thought and
that their masses are much larger (by >30 per cent) than inferred from
conventional, non-magnetic models in the Hertzsprung-Russell diagram.
Systematic changes of this size may be of great importance in understanding the
evolution of young stars, disc lifetimes and the formation of planetary
systems.
