Accurate estimates of fish age and growth are essential for stock assessment and management. We evaluated the differences in aging precision of Japanese sardines (Sardinops melanostictus) from the Sea of Japan and East China Sea determined from the counts of annual rings on scales and otolith observation by two readers. Changes in growth parameters between 2011 and 2018 year-classes were also evaluated. Although the aging precision between readers was relatively precise for both scales and otoliths, otoliths may be more useful for determining the age of this population because of the low discrepancy of the age counts between readers. The average standard length of Japanese sardines aged 2-3 years was relatively stable for 2011-2017 year-classes but declined sharply after 2018 year-class, along with increased stock biomass. Recent (2018 year-class) estimated theoretical asymptotic length also declined compared with those of early 2010s year-classes. Changes in the mean standard length at an age of 2-3 years and theoretical asymptotic length of Japanese sardines in the Sea of Japan and East China Sea may be a density-dependent response to increased stock abundance.