  To understand in what mass regime star-forming galaxies (SFGs) build up
central mass concentration most actively, we present a study on the
luminosity-weighted stellar age radial gradient ($\nabla_{\rm age}$)
distribution of $\sim3600$ low-redshift SFGs using the MaNGA Pipe3D data
available in the SDSS DR17. The mean age gradient is negative, with
$\nabla_{\rm age}=-0.14$log Gyr/$R_{\rm e}$, consistent with the inside-out
disk formation scenario. Specifically, SFGs with positive $\nabla_{\rm age}$
consist of $\sim 28\%$ at log$(M_{*}/M_{\odot})<9.5$, while this fraction rises
up to its peak ($\sim 40\%$) near log$(M_{*}/M_{\odot})=10$ and then decreases
to $\sim 15\%$ at log$(M_{*}/M_{\odot})=11$. At fixed $M_{*}$, SFGs with
positive $\nabla_{\rm age}$ typically have more compact sizes and more
centrally concentrated star formation than their counterparts, indicative of
recent central mass build-up events. These results suggest that the build-up of
central stellar mass concentration in local SFGs is mostly active near
$M_{*}=10^{10}M_{\odot}$. Our findings provide new insights on the origin of
morphological differences between low-mass and high-mass SFGs.
