  The surface rotation rates of young solar-type stars vary rapidly with age
from the end of the pre-main sequence through the early main sequence.
Important changes in the dynamos operating in these stars may result from this
evolution, which should be observable in their surface magnetic fields. Here we
present a study aimed at observing the evolution of these magnetic fields
through this critical time period. We observed stars in open clusters and
stellar associations of known ages, and used Zeeman Doppler Imaging to
characterize their complex magnetic large-scale fields. Presented here are
results for 15 stars, from 5 associations, with ages from 20 to 250 Myr, masses
from 0.7 to 1.2 solar masses, and rotation periods from 0.4 to 6 days. We find
complex large-scale magnetic field geometries, with global average strengths
from 14 to 140 G. There is a clear trend towards decreasing average large-scale
magnetic field strength with age, and a tight correlation between magnetic
field strength and Rossby number. Comparing the magnetic properties of our
zero-age main sequence sample to those of both younger and older stars, it
appears that the magnetic evolution of solar-type stars during the pre-main
sequence is primarily driven by structural changes, while it closely follows
the stars' rotational evolution on the main sequence.
