  We present an analysis of infrared (IR) echelle spectra of five stars in the
TW Hydrae Association (TWA). We model the Zeeman broadening in four
magnetic-sensitive \ion{Ti}{1} lines near $2.2 \mu$m and measure the value of
the photospheric magnetic field averaged over the surface of each star. To
ensure that other broadening mechanisms are properly taken into account, we
also inspect several magnetically insensitive CO lines near $2.3 \mu$m and find
no excess broadening above that produced by stellar rotation and instrumental
broadening, providing confidence in the magnetic interpretation of the width of
the \ion{Ti}{1} lines. We then utilize our results to test the relationship
between stellar magnetic flux and X-ray properties and compare the measured
fields with equipartition field values. Finally, we use our results and recent
results on a large sample of stars in Taurus to discuss the potential evolution
of magnetic field properties between the age of Taurus ($\sim$2 Myrs) and the
age of TWA ($\sim$10 Myrs). We find that the average stellar field strength
increases with age; however, the total unsigned magnetic flux decreases as the
stars contract onto the main-sequence.
