Do your adult students complain that their memory is not the same? This article about spaced repetition might be just the thing they need. There’s a lot of advice that is applicable both to paper flashcards and to learning apps like Quizlet; as a bonus, the author is quite good at metaphors:
You can think of learning as being kind of like building a brick wall; if you stack the bricks up too quickly without letting the mortar between each layer solidify, you’re not going to end up with a very good wall. Spacing our your learning allows that “mental mortar” time to dry.
(I still prefer forget-me-nots to stones, so the picture has both.)