Context: 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 highlight the presence of metal-poor stars in cold disc orbits, suggesting an ancient disc. This prompts a fundamental question: When did the Milky Way disc form, and did it originate as the thin disc or the larger velocity dispersion thick disc? Aims: We do a chrono-chemo-dynamical study of a large sample of stars with precise stellar parameters, focusing on the oldest stars, to decipher the assembly history of the MW discs. Methods: We investigated a sample of 565 606 stars with 6D phase space information and high-quality stellar parameters coming from the hybrid-CNN analysis of the Gaia-DR3 RVS stars. The sample contains 8500 stars with [Fe/H] <‑1. For a subset of ∼200 000 main sequence turnoff (MSTO) and subgiant branch (SGB) stars we computed distances and ages using the StarHorse code with a mean precision of 1% and 12%, respectively. Results: First, we confirm the existence of metal-poor stars in thin disc orbits. The majority of these stars are predominantly old, with over 50% being older than 13 Gyr. Second, we report the discovery of the oldest thin disc of the Milky Way extending 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. We calculate the vertical velocity dispersion (σVz) for the high-[α/Fe] thick disc as 35 km/s while the thin disc at same age range has a σVz lower by 10 to 15 km/s. Our old thin disc σVz appears similar to those estimated for the high-z disc galaxies. Third, as a verification of StarHorse ages, 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 of the old thin disc by showing that the Splash includes both old (> 9 Gyr) high- and low-[α/Fe] populations and extends to a wider [Fe/H] range reaching super-solar [Fe/H] . We find about 6 to 10% of the old thin disc was heated to thick disc orbits. The youngest splashed stars appear at 9 to 10 Gyrs and possibly hint to the GSE merger at this period. Conclusions: The Milky Way thin disc forms less than 1 billion years from Big Bang and continuously builds up in an inside out manner - this finding precedes the earlier estimates of start of the MW thin disc formation (around 8-9 Gyr) by about 4-5 billion years. We find that the metal-poor stars in disc orbits reported by previous studies belong to this old thin disc. 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.