In Sect.4, we present the fitting method and the derived ages of the sample galaxies. In Sect.5, the incompleteness effects are discussed. The discussions are given in Sect.6. We summarize and conclude the work in Sect.7. Throughout this paper, a cosmological model with $H_0$=70 km s$^{-1}$ Mpc$^{-1}$, $\Omega _M$=0.3 and $\Omega _\Lambda =0.7$ has been adopted. All the magnitudes are in AB system.

The sample {#sect:Obs}
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Our sample galaxies are selected by matching three databases from multiwavelength surveys, i.e., the SDSS for ugriz optical, 2MASS for JHKs NIR, and GALEX for FUV and NUV bands.

The SDSS sample
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The SDSS[^1] is the most ambitious astronomical survey ever undertaken in imaging and spectroscopy (York et al. 2000; Stoughton et al. 2002). The photometric and spectroscopic observations were conducted using the 2.5-m SDSS telescope at the Apache Point Observatory in New Mexico, USA. The imaging data were done in drift-scan mode in ugriz five bands, with effective central wavelengths of 3551, 4686, 6166, 7480 and 8932 Å, respectively (Gunn et al. 1998), and the 95% completeness limits for point sources are 22.0, 22.2, 22.2, 21.3 and 20.5 mag, respectively. The spectra are flux- and wavelength-calibrated with 4096 pixels from 3800 