  In this paper, we use the Type Ia supernova data as well as the CMB and LSS
data to constrain the agegraphic dark energy model recently proposed by Cai.
Due to its peculiar nature, the parameter $n$ of this model cannot be well
constrained by the SNIa data, while the other parameter $\Omega_{m0}$ can be
constrained to be $0.34\pm0.04$. When combined with CMB and LSS data, the range
of $1\sigma$ confidence level for $n$ is greatly narrowed, albeit still very
large. The best fit result is $\Omega_{m0}=0.28\pm0.02$, which is consistent
with most observations like WMAP and SDSS, and $n=3.4$, of which a meaningful
range of confidence level can not be obtained due to the fact that the contours
are not closed. Despite of this result, we conclude that for $n>1$ this model
is consistent with SNIa, CMB and LSS observations. Furthermore, the fitting
results indicate a generalized definition for the agegraphic dark energy.
