  Two eclipsing binaries in the USco association have recently yielded precise
values of masses and radii for 4 low-mass members of the association. Standard
evolution models would require these dM4.5 - dM5 stars to have ages which are
younger than the ages of more massive stars in the association by factors which
appear (in extreme cases) to be as large as ~3. Are the stars in the
association therefore non-coeval? We suggest that the answer is No: by
incorporating the effects of magnetic inhibition of convective onset, we show
that the stars in USco can be restored to coevality provided that the 4
low-mass member stars have vertical surface fields in the range 200 - 700 G.
Fields of such magnitude have already been measured on the surface of certain
solar-type stars in other young clusters.
