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The HST/WFC3 multiband photometry spanning from the UV to the near-IR of fourfields in the Galactic bulge, together with that for six template globular andopen clusters, are used to photometrically tag the metallicity [Fe/H] of starsin these fields after proper-motion rejecting most foreground diskcontaminants. Color-magnitude diagrams and luminosity functions are thenconstructed, in particular for the most metal rich and most metal poor stars ineach field. We do not find any significant difference between the $I$-band and$H$-band luminosity functions, hence turnoff luminosity and age, of the metalrich and metal poor components which therefore appear essentially coeval. Inparticular, we find that no more than $\sim 3\%$ of the metal-rich componentcan be $\sim 5$ Gyr old, or younger. Conversely, theoretical luminosityfunctions give a good match to the observed ones for an age of ~10 Gyr.Assuming this age is representative for the bulk of bulge stars, we then recallthe observed properties of star-forming galaxies at 10 Gyr lookback time, i.e.,at z~2, and speculate about bulge formation in that context. We argue that barformation and buckling instabilities leading to the observed boxy/peanut,X-shaped bulge may have arisen late in the history of the Milky Way galaxy,once its gas fraction had decreased compared to the high values typical ofhigh-redshift galaxies. This paper follows the public release of thephotometric and astrometric catalogs for the measured stars in the four fields.

The WFC3 Galactic Bulge Treasury Program: Relative Ages of Bulge Stars of High and Low Metallicity