  The first neutron star-neutron star merger was detected in by LIGO/Virgo in a
galaxy in which the majority of star formation was taking place a long time ago
(11 Gyr). LIGO/Virgo estimated that local cosmic NS-NS merger rate is 110-3840
Gpc^-3 yr^-1. Only some extreme evolutionary models can generate NS-NS merger
rates in old host galaxies consistent with the LIGO/Virgo estimate. However, we
show that these models generate rates exceeding empirical Galactic NS-NS merger
rates based on the large population of Milky Way NS-NS binaries. Typically,
current evolutionary models produce NS-NS merger rates that are consistent with
the Milky Way empirical rates. However, these models generate local cosmic
NS-NS merger rate in old host galaxies that are below the LIGO/Virgo estimate.
The reason behind this tension is the predicted delay time distribution between
star formation and NS-NS mergers that favors short delays. Evolutionary models
produce a generic steep power-law NS-NS delay time distribution. This limits
NS-NS merger rates in old host galaxies. However, we show that such
distribution is consistent with observations of Galactic NS-NS binaries; 50% of
which show very long merger times (much longer than Hubble time). Once model
distributions are convolved with continuous prolonged (10 Gyr) star formation
in the Galactic disk, then 20-70% (depending on a model) of the predicted NS-NS
population has very long current Galactic merger times. Although NS-NS binaries
are formed predominantly with short delay times, many of short delay time
systems merge and do not make it to the present, while long delay time systems
survive and contribute to the current Galactic NS-NS population. This study
highlights the tension between the current evolutionary predictions and the
observation of the first NS-NS merger in an old host galaxy.
