When someone gets charged with a criminal offense, they have the right to defend themselves in court. Depending on the circumstances, there are a number of strategies that can help someone prove that they didn't commit the crime in question.
Imagine some of the things you did as a teen. Perhaps some of the things you did were illegal or dangerous. While some people don't get caught, others do. They might end up in prison for crimes others got away with. Now, years later, they're still doing time for a crime they committed when they were young and less informed than today.