There are two main reasons why criminal activity drops after young people pass through the period of adolescence. These reasons are connected, but separate. One has to do with the obligations that they come to have after adolescence and the other has to do with the change in their psychology after that stage.
Psychologists tell us that adolescence is a time of pushing boundaries and trying to establish identities. Adolescents are eager to determine what their true identity is. In doing so, they need to push back against the norms and values that they have learned from authority figures. This is one reason why adolescents are likely to commit crimes. They want to experiment with identities in which they see themselves as rebels. They want to establish their independence and the fact that they do not have to behave in the way that society expects them to. Because of these things, they commit crimes. Once they have passed through adolescence, they no longer feel these urges as strongly and are less likely to commit crimes.
We also know that adolescents have fewer responsibilities in their lives. This lack of responsibility can make crime more feasible for them. They do not have to go to jobs on a regular basis. They do not have families that expect them to be home at certain times. This lack of responsibility gives them the time and the psychological “space” to allow them to commit crimes. When they are through adolescence, young adults have jobs and responsibilities and have less free time on their hands in which to commit crimes.
For these two reasons, we typically see decreases in crime rates after people pass through adolescence.