  The initial condition $\Omega_{\rm de}(z_{\rm ini})=n^2(1+z_{\rm
ini})^{-2}/4$ at $z_{\rm ini}=2000$ widely used to solve the differential
equation of the density of the new agegraphic dark energy (NADE) $\Omega_{\rm
de}$, makes the NADE model be a single-parameter dark-energy cosmological
model. However, we find that this initial condition is only applicable in a
flat universe with only dark energy and pressureless matter. In fact, in order
to obtain more information from current observational data, such as cosmic
microwave background (CMB) and baryon acoustic oscillations (BAO), we need to
consider the contribution of radiation. For this situation, the initial
condition mentioned above becomes invalid. To overcome this shortage, we
investigate the evolution of dark energy in the matter-dominated and
radiation-dominated epochs, and obtain a new initial condition $\Omega_{\rm
de}(z_{\rm ini})=\frac{n^2(1+z_{\rm ini})^{-2}}{4}(1+\sqrt{F(z_{\rm ini})})^2$
at $z_{\rm ini}=2000$, where
$F(z)\equiv\frac{\Omega_{r0}(1+z)}{\Omega_{m0}+\Omega_{r0}(1+z)}$ with
$\Omega_{r0}$ and $\Omega_{m0}$ being the current density parameters of
radiation and pressureless matter, respectively. This revised initial condition
is applicable for the differential equation of $\Omega_{\rm de}$ obtained in
the standard Friedmann-Robertson-Walker (FRW) universe with dark energy,
pressureless matter, radiation, and even spatial curvature, and can still keep
the NADE model being a single-parameter model. With the revised initial
condition and the observational data of type Ia supernova (SNIa), CMB and BAO,
we finally constrain the NADE model. The results show that the single free
parameter $n$ of the NADE model can be constrained tightly.
