  We investigate the stellar populations of Lyman alpha emitters (LAEs) at
z=5.7 and 6.6 in a 0.65 deg^2 sky of the Subaru/XMM-Newton Deep Survey (SXDS)
Field, using deep images taken with Subaru/Suprime-Cam, UKIRT/WFCAM, and
Spitzer/IRAC. We produce stacked multiband images at each redshift from 165
(z=5.7) and 91 (z=6.6) IRAC-undetected objects, to derive typical spectral
energy distributions (SEDs) of z~6-7 LAEs for the first time. The stacked LAEs
have as blue UV continua as the HST/WFC3 z-dropout galaxies of similar Muv,
with a spectral slope beta ~ -3, but at the same time they have red
UV-to-optical colors with detection in the 3.6um band. Using SED fitting we
find that the stacked LAEs have low stellar masses of ~(3-10)*10^7 Msun, very
young ages of ~1-3 Myr, negligible dust extinction, and strong nebular emission
from the ionized interstellar medium, although the z=6.6 object is fitted
similarly well with high-mass models without nebular emission; inclusion of
nebular emission reproduces the red UV-to-optical color while keeping the UV
color sufficiently blue. We infer that typical LAEs at z~6-7 are building
blocks of galaxies seen at lower redshifts. We find a tentative decrease in the
Lyman alpha escape fraction from z=5.7 to 6.6, which may imply an increase in
the intergalactic medium neutral fraction. From the minimum contribution of
nebular emission required to fit the observed SEDs, we place an upper limit on
the escape fraction of ionizing photons to be f_esc^ion~0.6 at z=5.7 and ~0.9
at z=6.6. We also compare the stellar populations of our LAEs with that of
stacked HST/WFC3 z-dropout galaxies.
