Dependence of the predicted emission-line flux ratios ([Oiii]51.80+[Oiii]88.33)/[Niii]57.21 (upper panel) and [Nii]121.7/ [Niii]57.21 (lower panel) on the age of the stellar population used in the photoionization model calculations. Models with constant star-formation history and with solar metallicity are displayed. The meanings of lines are the same as in Fig. 3. In the lower panel, models with n_H = 10³ cm^-3 and U = 10^-1.5 are not plotted, because the flux of the [Nii]121.7 is too faint for those parameters. The displayed ranges of emission-line flux ratios in the upper and lower panels are the same as those in Fig. 5 and the lower panel of Fig. 6, respectively.