  We present resolved stellar photometry of 4 fields along the major axis of
the M33 disk from images taken with the Advanced Camera for Surveys aboard the
Hubble Space Telescope. The photometry provides a detailed census of the red
clump in all fields and reaches the ancient main sequence in the outermost
field. Through detailed modeling of the color-magnitude diagrams, we find that
the percentage of the stellar mass formed prior to z=1 changes from 71 +/- 9%
in the innermost field to 16 +/- 6% in the outermost field. The disk shows a
clear trend of increasing scale-length with time, evolving from 1.0 +/- 0.1 kpc
10 Gyr ago to 1.8 +/- 0.1 kpc at times more recent than 5 Gyr ago, in agreement
with analytical predictions for disk growth. Beyond the disk truncation radius,
however, the stellar density profile steepens with time and the age gradient
reverses, in agreement with recent simulations. The late and slow growth of the
stellar disk may be due in part to the low mass of M33.
