The pace and pattern of star formation leading to rich young stellar clusters is quite uncertain .
In this context , we analyze the spatial distribution of ages within 19 young ( median t \la 3 Myr on the Siess et al .
( 2000 ) timescale ) , morphologically simple , isolated , and relatively rich stellar clusters .
Our analysis is based on young stellar object ( YSO ) samples from the MYStIX and SFiNCs surveys , and a new estimator of pre-main sequence ( PMS ) stellar ages , Age _ { JX } , derived from X-ray and near-infrared photometric data .
Median cluster ages are computed within four annular subregions of the clusters .
We confirm and extend the earlier result of Getman et al .
( 2014 ) : 80 % percent of the clusters show age trends where stars in cluster cores are younger than in outer regions .
Our cluster stacking analyses establish the existence of an age gradient to high statistical significance in several ways .
Time scales vary with the choice of PMS evolutionary model ; the inferred median age gradient across the studied clusters ranges from 0.75 Myr pc ^ { -1 } to 1.5 Myr pc ^ { -1 } .
The empirical finding reported in the present study – late or continuing formation of stars in the cores of star clusters with older stars dispersed in the outer regions – has a strong foundation with other observational studies and with the astrophysical models like the global hierarchical collapse model of Vázquez-Semadeni et al .
( 2017 ) .