  In systems undergoing starbursts the evolution of the young stellar
population is expected to drive changes in the emission line properties. This
evolution is usually studied theoretically, with a combination of evolutionary
synthesis models for the spectral energy distribution of starbursts and
photoionization calculations. In this paper we present a more empirical
approach to this issue. We apply empirical population synthesis techniques to
samples of Starburst and HII galaxies in order to measure their evolutionary
state and correlate the results with their emission line properties. A couple
of useful tools are introduced which greatly facilitate the interpretation of
the synthesis: (1) an evolutionary diagram, whose axis are the strengths of the
young, intermediate age and old components of the stellar population mix, and
(2) the mean age of stars associated with the starburst, $\ov{t}_{SB}$. These
tools are tested with grids of theoretical galaxy spectra and found to work
very well even when only a small number of observed properties (absorption line
equivalent widths and continuum colors) is used in the synthesis.
  Starburst nuclei and HII galaxies are found to lie on a well defined sequence
in the evolutionary diagram. Using the empirically defined mean starburst age
in conjunction with emission line data we have verified that the equivalent
widths of H$\beta$ and [OIII] decrease for increasing $\ov{t}_{SB}$. The same
evolutionary trend was identified for line ratios indicative of the gas
excitation, although no clear trend was identified for metal rich systems. All
these results are in excellent agreement with long known, but little tested,
theoretical expectations.
