As any parent can attest, teenage brains are not fully formed. It isn't until about the age of 25 that the human brain reaches full maturity.
It doesn't matter how smart teens are or how well they scored on the SAT or ACT. Good judgment isn't something they can excel in, at least not yet.
The rational part of a teen's brain isn't fully developed and won't be until age 25 or so.
Recent research has found that adult and teen brains work differently. Adults think with the prefrontal cortex, the brain's rational part. This is the part of the brain that responds to situations with good judgment and an awareness of long-term consequences. Teens process information with the amygdala. This is the emotional part.
In teen's brains, the connections between the emotional part of the brain and the decision-making center are still developing—and not always at the same rate. That's why when teens have overwhelming emotional input, they can't explain later what they were thinking. They weren't thinking as much as they were feeling.