In addition to all the physical and emotional changes teens go through, they will also start to think differently. The biggest cognitive leap forward is that teens will begin thinking abstractly about things in ways that children simply cannot do. Because they can start to think abstractly, teens will begin to be able to consider multiple points of view, conduct reasoning from principles and ponder a full range of possibilities.
On a practical level, this means that teens will start to challenge adults more as they begin to learn to reason, argue and respond. They should also become better able to forecast and consider the consequences of actions before those actions occur. While you’d think this would lead to better decision making (and it does to a point), you should also realize that teens have these new problem-solving capabilities without life experience or context. So while teens can better understand and try to forecast consequences, they likely don’t have enough experience to forecast them accurately yet.
Teens’ abilities to question and ponder will also lead them to start to question their own identity. They will begin to understand that they play different roles to different people, such as child, sibling, student, athlete, etc. They’ll take these different roles, their new willingness to explore other options and try on various personas. Testing out new identities is a typical and necessary part of adolescence.
Not all these cognitive changes happen at once. In early adolescence, you’ll notice teens begin to question authority and express personal opinions about their own life. Once in the middle of adolescence, teens will then begin start to think about the future, start making goals and become more involved in forging their identity. In late adolescence, teens will start forming firm opinions about external issues, often quite idealistically. Their lack of context and experience will tend to make them fairly intolerant of opposing views. They will also begin to explore what their identity will be once they reach adulthood.