  Long-slit Keck II, 4m Kitt Peak, and 4.5m MMT spectrophotometric data are
used to investigate the stellar population and the evolutionary status of I Zw
18C, the faint C component of the nearby blue compact dwarf galaxy I Zw 18.
Hydrogen H$\alpha$ and H$\beta$ emission lines are detected in the spectra of I
Zw 18C, implying that ionizing massive stars are present. High signal-to-noise
Keck II spectra of different regions in I Zw 18C reveal H$\gamma$, H$\delta$
and higher order hydrogen lines in absorption. Several techniques are used to
constrain the age of the stellar population in I Zw 18C. Ages derived from two
different methods, one based on the equivalent widths of the H$\alpha$,
H$\beta$ emission lines and the other on H$\gamma$, H$\delta$ absorption lines
are consistent with a 15 Myr instantaneous burst model. We find that a small
extinction in the range $A_V$ = 0.20 -- 0.65 mag is needed to fit the observed
spectral energy distribution of I Zw 18C with that model. In the case of
constant star formation, all observed properties are consistent with stars
forming continuously between ~ 10 Myr and < 100 Myr ago. We use all available
observational constraints for I Zw 18C, including those obtained from Hubble
Space Telescope color-magnitude diagrams, to argue that the distance to I Zw 18
should be as high as ~ 15 Mpc. The deep spectra also reveal extended ionized
gas emission around I Zw 18. H$\alpha$ emission is detected as far as 30" from
it. To a B surface brightness limit of ~ 27 mag arcsec$^{-2}$ we find no
observational evidence for extended stellar emission in the outermost regions,
at distances > 15" from I Zw 18.
