I've found that students who just came from high school have a sense of structure that is still a bit ingrained in their thinking, and therefore can do well in my classes. I also find that students over 30 are serious about learning and getting a good career. But the in-between years are the hardest. Many students who are 23 or 24 have been away from school for a while. They tend to have the hardest time getting back into a structured environment, and they aren't yet serious about a 'career'. It just takes a bit more maturity than they sometimes seem to muster up!