The J - K _ { s } color distribution ( CD ) with a bin size of 0.05 magnitude for the entire Milky Way has been carried out by using the Two Micron All Sky Survey Point Source Catalog ( 2MASS PSC ) .
The CDs are bimodal , which has a red peak at 0.8 < J - K _ { s } < 0.85 and a blue peak at 0.3 < J - K _ { s } < 0.4 .
The colors of the red peak are more or less the same for the whole sky , but that of the blue peak depend on Galactic latitude ( J - K _ { s } \sim 0.35 at low Galactic latitudes and 0.35 < J - K _ { s } < 0.4 for other sky areas ) .
The blue peak dominates the bimodal CDs at low Galactic latitudes and becomes comparable with the red peak in other sky regions .
In order to explain the bimodal distribution and the global trend shown by the all sky 2MASS CDs , we assemble an empirical HR diagram , which is composed by observational-based near infrared HR diagrams and color magnitude diagrams , and incorporate a Milky Way model .
In the empirical HR diagram , the main sequence stars turnoff of the thin disk is relatively bluer , ( J - K _ { s } ) _ { 0 } = 0.31 , when we compare with the thick disk which is ( J - K _ { s } ) _ { 0 } = 0.39 .
The age of the thin/thick disk is roughly estimated to be around 4-5/8-9 Gyr according to the color-age relation of the main sequence turnoff .
In general , the 2MASS CDs can be treated as a tool to census the age of stellar population of the Milky Way in a statistical manner and to our knowledge this is a first attempt to measure the age .