  Integrated spectroscopy is the method of choice for deriving the ages of
unresolved stellar systems. However, hot stellar evolutionary stages, such as
hot horizontal branch stars and blue straggler stars (BSSs), can affect the
integrated ages measured using Balmer lines. Such hot, "non-canonical" stars
may lead to overestimations of the temperature of the main sequence turn-off,
and therefore underestimations of the integrated age of a stellar population.
Using an optimized Hbeta index in conjunction with HST/WFPC2 color-magnitude
diagrams (CMDs), we show that Galactic globular clusters exhibit a large
scatter in their apparent "spectroscopic" ages, which does not correspond to
that in their CMD-derived ages. We find for the first time that the specific
frequency of BSSs, defined within the same aperture as the integrated spectra,
shows a clear correspondence with Hbeta in the sense that, at fixed
metallicity, higher BSS ratios lead to younger "apparent" spectroscopic ages.
Thus, the specific frequency of BSSs in globular clusters sets a fundamental
limit on the accuracy for which spectroscopic ages can be determined for
globular clusters, and maybe for other stellar systems like galaxies. The
observational implications of this result are discussed.
