DD1 started classes 2 years ago, and she's 5.5 now. To answer your question
about what age she began to pay closer attention, I'd say that would be 4! (And of course
even better at 4.5 and now 5).
One thing she's gotten out of the classes is a chance to *focus* her spirited energy,
and gain confidence trying new things and accomplishing them. As well as learning
about patience, when to be quiet and listen to the teacher, (she was just starting preschool)