In the period from 4 days post-hatching to metamorphosis, winter flounder that were naïve to artificial feed were exposed to an artificial diet and allowed to forage for 8 min. The presence or absence of artificial diet in the gut was used as an indicator of acceptance. The relationship between diet acceptance and age was found to be a dome-shaped function (incidence of ingested food = −0.0233 day2 + 1.101 day − 4.9428, r2 = 0.85). Here, our analysis suggests that winter flounder should be exposed to artificial diets by 26 days post-hatch at 10 °C, after this time receptivity declines. This age corresponds to larvae that are in the flexion developmental stage and have a standard length between 5.95 and 6.23 mm.