  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,
AgeJX. Stellar masses are derived from X-ray luminosities using the Lx - Mass
relation from the Taurus cloud. J-band luminosities are compared to
mass-dependent pre-main-sequence evolutionary models to estimate ages. AgeJX 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 AgeJX 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 AgeJX 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.
