et al. 2006). Blanton et al. (2005) had nicely discussed the incompleteness at low surface brightness, and presented the contributions to completeness as a function of surface brightness, the $r$-band Petrosian half-light surface brightness $\mu_{50,r}$. It shows that, for those brighter ones with $\mu_{50,r}<$23 mag arcsec$^{-2}$ (corresponding to $\mu_{0}$(B)=24.5 mag arcsec$^{-2}$), the completeness of the spectroscopy is very close to 100%. But for the photometric and tilling catalogs, they show obvious incompleteness within $\mu_{50,r}$=22-23 mag arcsec$^{-2}$, then the total completeness there could decrease to be about 70%.

Similar to Zhong et al. (2008) in their Sect.5, here we also extract a volume-limited sub-sample to minimize the effects of sample incompleteness on the derived ages of galaxies. We firstly extract a volume-limited sub-sample from the $M_r-z$ plane by considering $z<0.1$ and those brighter ones than the corresponding $M_r$ from the SDSS-DR7 MGS as given in Sect. \[sect:Obs\].1. We obtain 5,608 LSBGs and 8,046 HSBGs. Then, we match them with our sample galaxies having FUV to NIR SEDs (i.e., the 1,802 LSBGs and 5,886 HSBGs). This allows us to obtain a sub-sample including 1,046 LSBGs and 2,853 HSBGs. Their ages are given in Fig. \[fig.Mr.z\] as histogram distributions. The left panel is 