A major impediment to understanding star formation in massive star forming regions ( MSFRs ) is the absence of a reliable stellar chronometer to unravel their complex star formation histories .
We present a new estimation of stellar ages using a new method that employs near-infrared ( NIR ) and X-ray photometry , Age _ { JX } .
Stellar masses are derived from X-ray luminosities using the L _ { X } - M relation from the Taurus cloud . J -band luminosities are compared to mass-dependent pre-main-sequence evolutionary models to estimate ages . Age _ { JX } is sensitive to a wide range of evolutionary stages , from disk-bearing stars embedded in a cloud to widely dispersed older pre-main sequence stars .
The MYStIX ( Massive Young Star-Forming Complex Study in Infrared and X-ray ) project characterizes 20 OB-dominated MSFRs using X-ray , mid-infrared , and NIR catalogs .
The Age _ { JX } method has been applied to 5525 out of 31,784 MYStIX Probable Complex Members .
We provide a homogeneous set of median ages for over a hundred subclusters in 15 MSFRs ; median subcluster ages range between 0.5 Myr and 5 Myr .
The important science result is the discovery of age gradients across MYStIX regions .
The wide MSFR age distribution appears as spatially segregated structures with different ages .
The Age _ { JX } ages are youngest in obscured locations in molecular clouds , intermediate in revealed stellar clusters , and oldest in distributed populations .
The NIR color index J - H , a surrogate measure of extinction , can serve as an approximate age predictor for young embedded clusters .