Many students will have graduated by the time they are 24, so it is possible that both statements 1. the majority of American children go to college and 2. 40 percent of 18-24 year olds are enrolled are simultaneously true
I believe they are actually measuring any time enrollment in that period. Otherwise the statistic seems of little value.
On time high school graduation is achieved by around 86% of the population. This surely accounts for much of the difference.