  Globular cluster ages provide both an important test of models of globular
cluster formation and a powerful method to constrain the assembly history of
galaxies. Unfortunately, measuring the ages of unresolved old stellar
populations has proven challenging. Here, we present a novel technique that
combines optical photometry with metallicity constraints from near-infrared
spectroscopy in order to measure ages. After testing the method on globular
clusters in the Milky Way and its satellite galaxies, we apply our technique to
three massive early-type galaxies using data from the SLUGGS Survey. The three
SLUGGS galaxies and the Milky Way show dramatically different globular cluster
age and metallicity distributions, with NGC 1407 and the Milky Way showing
mostly old globular clusters while NGC 3115 and NGC 3377 show a range of
globular ages. This diversity implies different galaxy formation histories and
that the globular cluster optical colour-metallicity relation is not universal
as is commonly assumed in globular cluster studies. We find a correlation
between the median age of the metal rich globular cluster populations and the
age of the field star populations, in line with models where globular cluster
formation is a natural outcome of high intensity star formation.
