Defining "adult learners" isn't easy. After all, youths in Japan can vote and drink at the age of 20, yet when attending university they are not considered adults.
In international studies in the past, students (gakusei) were often classed by age, with those over 24 or 25 considered "adult" or "mature" learners. More recently, those terms have been replaced by "nontraditional student," which can embrace a range of factors, from age, employment and family status to whether or not a student lives on campus.
It's both a more complex category, and one that reflects reality better than age alone.
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