Why do females generally need fewer calories than males?
The main concept here is "caloric balance", the point at which your energy intake and your energy expenditure are equal. If you consume more energy that you expend, you gain weight, and if you expend more energy than you consume you lose weight. For most people, the ideal is to remain in caloric balance, i.e. stay the same weight.
How many calories you expend depends on two main things, basal metabolic rate and activity level. Your BMR -- the number of calories you would use just lying in bed all day -- is a function of lean body mass, percentage of body fat, age, temperature, height and physical condition. Because men generally weigh more than women and have slightly lower body fat, they tend to have a slightly higher BMR. On the other hand, a male couch potato may have a lower BMR than a female athlete.
Your BMR declines as you age as well. Adolescence is a period when you have the highest energy needs because of a high BMR, a usually high activity level, and the needs of your body as it is still growing.
The number of daily calories ingested has absolutely nothing to do with gender. The main factor for caloric needs is the persons activity level. The more calories a person burns means that they must consume additional calories to replace the ones used by the body. Athlete's of any gender are good examples. Because they use many more calories than the average person they must consume more to replenish.