  As the third paper of our serial studies that are aim at examining the
AGN-host coevolution by using partially obscured AGNs, we extend the broad-line
composite galaxies (composite AGNs) into ROSAT-selected Seyfert 1.8/1.9
galaxies basing upon the RASS/SDSS-DR5 catalog given by Anderson et al.. The
SDSS spectra of in total 92 objects are analyzed by the same method used in our
previous studies, after requiring the signal-to-noise ratio in the SDSS r' band
is larger than 20. Combing the ROSAT-selected Seyfert galaxies with the
composite AGNs reinforces the tight correlation between the line ratio
[OI]/H\alpha vs. D_n(4000), and establishes a new tight correlation between
[SII]/H\alpha vs. D_n(4000). Both correlations suggest the two line ratios are
plausible age indicators of the circumnuclear stellar population for typical
type I AGNs in which the stellar populations are difficult to be derived from
their optical spectra. The ROSAT-selected Seyfert galaxies show that the two
correlations depend on the soft X-ray spectral slope \alpha_X that is roughly
estimated from the hardness ratios by requiring the X-ray count rates within
0.1-2.4 keV are larger than 0.02 counts s^-1. However, we fail to establish a
relationship between \alpha_X and D_n(4000), which is likely caused by the
relatively large uncertainties of both parameters (especially for \alpha_X
because of the AGN intrinsic obscuration). The previously established
L/L_Edd-D_n(4000) evolutionary sequence is reinforced again by the extension to
the ROSAT-selected Seyfert galaxies. These X-ray-selected Seyfert galaxies are,
however, biased against the two ends of the sequence, which implies that the
X-ray Seyfert galaxies present a population at middle evolutionary stage.
