Is a 5' 8 170 pound 14-year-old overweight?

It would depend on your BMI- your body mass index.

Being 15, I understand that you could reach 180 pounds in several different ways. If the weight is mostly muscular, then it's fine. If it's mostly fat, I'd be concerned.

Of course, it could be neither. People who are taller usually weigh more than shorter people.