Why does a teen throw fits at 17 years old?

Because it works: The fitting behavior that should have disappeared after the development of competent speech is a sign of regression. It obviously works on someone or it would have long since subsided.A few sessions with a family counselor involving all those involved is in order. After initial evaluation one or more of you will need work to develop new response patterns.

Adolescence : Teens are still learning patience, wisdom & self control. There's never been a teen who hasn't had a rage or meltdown at some point during that learning process. If the "fits" are getting out of hand or are associated with conduct or behavioral problems outside the home, then an intervention may be necessary. Start by talking to your pediatrician.