The surface rotation rates of young solar-type stars decrease rapidly with age from the end of the pre-main sequence though the early main sequence .
This suggests that there is also an important change in the dynamos operating in these stars , which should be observable in their surface magnetic fields .
Here we present early results in a study aimed at observing the evolution of these magnetic fields through this critical time period .
We are observing stars in open clusters and stellar associations to provide precise ages , and using Zeeman Doppler Imaging to characterize the complex magnetic fields .
Presented here are results for six stars , three in the in the \beta Pic association ( \sim 10 Myr old ) and three in the AB Dor association ( \sim 100 Myr old ) .