Is 160 pounds an average weight for a twelve year old who is 5 feet 7 inches and a half?

No. The child is overweight. Most 12 year olds who are 5'7" weigh less than this. You didn't say if it is a girl or a boy and what the frame size of the child is, but a registered nurse can tell you how much weight the child needs to lose. A child who is more muscular will weigh more because muscle weighs more than fat. Any child who goes on a diet needs to have guidance from a health care professional. The best way to lose weight is to eat a well-balanced diet but eat smaller servings, and to exercise at least 45 minutes three times a week to the point that the heart beats fast. It also helps to eat food as close as possible to its natural state. For example, eat an Apple instead of Apple juice or instead of sweetened Apple sauce.