robably the current best indicator of a healthy weight is the BMI, which calculates the percent body fat. General guidelines are posted on the Healthy Body Calculator® BMI graph. A BMI < 25 is considered healthy, 26 - 29 overfat and > 30 unhealthy, but is age dependent. I would suggest you surf the net for Body Mass Index or BMI to get more information.
As to how to get to a healthy BMI, try diet and exercise. Diet to reduce weight. Exercises to increase muscle mass and decrease fat. Muscle burns many times the calories that fat does and keeps your metabolic rate higher for up to 15 hours after exercise. Basically, body fat just sits there, occupying space and storing calories. A certain amount of body fat is necessary though to produce sex hormones, insulate the body from cold and cushion the internal organs.
A healthy weight and muscle mass varies between individuals and depends on gender, height, weight and age. However, there is no reason that with increasing age, a person can’t maintain a healthy weight through diet and exercise.