What should a 31 year old man that is 6 feet tall weigh?

It depends on all kinds of variables... Are you muscular or obese? A 6'3 athlete is going to weigh differently than a 6'3 coach potato. What is your body build? Do you work out? Are you a healthy eater? The Body Mass Index scale system is too general and gives a "what you should weigh" approach based on a linear graph based on age/weigh/weight. Not acurate at all!