  Simulating thin and extended galactic disks has long been a challenge in
computational astrophysics. We introduce the NIHAO-UHD suite of cosmological
hydrodynamical simulations of Milky Way mass galaxies and study stellar disk
properties such as stellar mass, size and rotation velocity which agree well
with observations of the Milky Way and local galaxies. In particular, the
simulations reproduce the age-velocity dispersion relation and a
multi-component stellar disk as observed for the Milky Way. Half of our
galaxies show a double exponential vertical profile, while the others are well
described by a single exponential model which we link to the disk merger
history. In all cases, mono-age populations follow a single exponential whose
scale height varies monotonically with stellar age and radius. The scale length
decreases with stellar age while the scale height increases. The general
structure of the stellar disks is already set at time of birth as a result of
the inside-out and upside-down formation. Subsequent evolution modifies this
structure by increasing both the scale length and height of all mono-age
populations. Thus, our results put tight constraints on how much dynamical
memory stellar disks can retain over cosmological timescales. Our simulations
demonstrate that it is possible to form thin galactic disks in cosmological
simulations provided there are no significant stellar mergers at low redshifts.
Most of the stellar mass is formed in-situ with only a few percent
($\lesssim5\%$) brought in by merging satellites at early times. Redshift zero
snapshots and halo catalogues are publicly available.
