The HST/WFC3 multiband photometry spanning from the UV to the near-IR of four fields in the Galactic bulge , together with that for six template globular and open clusters , are used to photometrically tag the metallicity [ Fe/H ] of stars in these fields after proper-motion rejecting most foreground disk contaminants .
Color-magnitude diagrams and luminosity functions are then constructed , in particular for the most metal rich and most metal poor stars in each field .
We do not find any significant difference between the I -band and H -band luminosity functions , hence turnoff luminosity and age , of the metal rich and metal poor components which therefore appear essentially coeval .
In particular , we find that no more than \sim 3 \% of the metal-rich component can be \sim 5 Gyr old , or younger .
Conversely , theoretical luminosity functions give a good match to the observed ones for an age of \sim 10 Gyr .
Assuming this age is representative for the bulk of bulge stars , we then recall the observed properties of star-forming galaxies at 10 Gyr lookback time , i.e. , at z \sim 2 , and speculate about bulge formation in that context .
We argue that bar formation 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 of high-redshift galaxies .
This paper follows the public release of the photometric and astrometric catalogs for the measured stars in the four fields .