the observed SEDs following $$\chi^2=\frac{1}{d}\sum^{10}_{i=1}\frac{[m^{obs}_{\lambda_i}-m^{mod}_{\lambda_i}(t)]^2}{\sigma^2_{i}},$$ where $m^{mod}_{\lambda_i}(t)$ is the integrated magnitude in the $i$th filter of a theoretical SED at age $t$, $m^{obs}_{\lambda_i}(t)$ represents the observed integrated magnitude in the same filter, and $\sigma_i^2$ is the observational uncertainty for the $i$th filter magnitude, $d$ is the number of degrees of freedom. We did not consider the uncertainty associated with the model itself here, which should be insignificant and not affect much our results. More details about the fitting method can be referred to Li et al. (2004a,b) and Ma et al. (2009a,b). The derived ages of our sample galaxies from such SED fitting analyses are given in next two subsections.

Constant dust extinction {#sect:analysis1}
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We adopt constant $A_V$=0.6 mag for LSBGs and $A_V$=0.8 mag for HSBGs in fittings by following Liang et al. (2010). In Liang et al. (2010), we derive the $A_V$ value for each object from their spectroscopic Balmer decrement H$\alpha$/H$\beta$ (see their Fig.2 for histogram distributions). The median values of $A_V$ of the four subsamples (with $\mu_0(B)$ in units of mag arcsec$^{-2}$: vLSBGs with 22.75-24.5, iLSBGs with 22.0-22.75, iHSBGs with 21.25-22.0, vHSBGs with $<$21.25) are 0.46, 0.63, 0.76, 0.83, respectively. In our present work, most LSBGs 