  We have defined a new H_\beta absorption index definition, H$_{\beta_o}$,
which has been optimized as an age indicator for old and intermediate age
stellar populations. Rather than using stellar spectra, we employed for this
purpose a library of stellar population SEDs of different ages and
metallicities at moderately high spectral resolution. H$_{\beta_o}$ provides us
with improved abilities for lifting the age-metallicity degeneracy affecting
the standard Hbeta Lick index definition. The new index, which has also been
optimized against photon noise and velocity dispersion, is fully characterized
with wavelength shift, spectrum shape, dust extinction and [alpha/Fe] abundance
ratio effects. H_\beta requires spectra of similar qualites as those commonly
used for measuring the standard Hbeta Lick index definition. Aiming at
illustrating the use and capabilities of H$_{\beta_o}$ as an age indicator we
apply it to Milky Way globular clusters and to a well selected sample of
early-type galaxies covering a wide range in mass. The results shown here are
particularly useful for applying this index and understand the involved
uncertainties.
