  JWST has recently detected numerous disc galaxies at high-redshifts and there
have been observations of cold disc galaxies at z > 4 with ALMA. In the Milky
Way, recent studies find metal-poor stars in cold disc orbits, suggesting an
ancient disc. We investigated a sample of 565,606 stars from the hybrid-CNN
analysis of the Gaia-DR3 RVS stars. The sample contains 8,500 stars with
[Fe/H]<-1. For a subset of ~200,000 MSTO and subgiant stars we computed
distances and ages using the StarHorse code with a mean precision of 1% and
12%, respectively. First, we confirm the existence of metal-poor stars in thin
disc orbits - over 50% are older than 13 Gyr. Second, we report the discovery
of the oldest thin disc of the Milky Way(MW), which extends across a wide range
of metallicities, from metal-poor to super-solar stars. The metal-poor stars in
disc orbits manifest as a readily visible tail of the metallicity distribution.
The high-[{\alpha}/Fe] thick disc exhibits a vertical velocity dispersion of 35
km/s, while the thin disc shows 10 to 15 km/s lower at similar ages. Our old
thin disc $\sigma_{V_z}$ appears similar to those estimated for the high-z disc
galaxies. Third, we extend the [Y/Mg] chemical clock to the oldest ages and
estimate a slope of -0.038 dex/Gyr. Finally, we confirm our discovery by
showing that the splash includes high- and low-[{\alpha}/Fe] populations that
are both old and extends to super-solar [Fe/H]. We find about 6 to 10% of the
old thin disc was heated to thick disc orbits with the youngest splashed stars
being 9 to 10 Gyrs. We conclude the MW thin disc forms <1 billion years from
Big Bang, building up inside-out, preceding earlier estimates by about 4-5
billion years. Considering a massive merger event such as the GSE, a Splash is
expected - we find a portion of the old thin disc is heated to thick disc
velocities and the Splash extends to super-solar [Fe/H] regimes.
