Research suggests that gains in executive function (EF) skills training are strongest when task difficulty increases progressively, yet findings on the effectiveness of adaptive approaches for EF training are inconsistent. This study compared the effectiveness of an adaptive vs a non-adaptive version of a digital game designed to train the EF sub-skill of shifting. Results showed increases in shifting skills for all learners between pretest and posttest measures, with adolescents scoring higher than pre-adolescents and early adolescents on posttest measures. Data analysis uncovered a trend suggesting that the adaptive treatment may be more effective than the non-adaptive treatment for adolescents. User logs showed that adaptivity helped customize players' gameplay based on their performance, by making game play easier for younger learners, and making game play more difficult for older learners. Results support the use of digital games to train EF for a broad range of learners. [This paper was published in "Cognitive Development" v49 p56-67 2019.]