The best boxer at my gym is 16 year's old and 5'7" 140lbs. He isn't very defined at all, but doesn't really have any fat. I guess you could describe his physique as fit.
He is about 6 ft tall 178 pounds, much quicker than any 150-200 pounder at the gym and trains the most there, some say he is a talented boxer. Ohh wait... I just described myself. :)