Measuring Daily Caloric Intake

       When nutrients are used in the body for energy, the amount of potential energy they release is measured in calories. One calorie is the amount of energy required to raise the temperature of on gram of water by 1 degree Celsius. However, food contains so much potential energy that scientists developed an ingenius term for this dietary unit of energy — the Calorie, with a capital "C", equal to 1,000 calories (one Calorie raises one liter of water by 1 degree Celsius). The dietary Calorie is used so often that we now simply refer to it as a "calorie". The caloric intake for teenagers varies from person to person. There are several factors that can change it.

Gender
       The average boy needs 2,100 to 2,400 calories per day. The average girl needs around 1,700 calories per day. Why is there a difference? Boys have a higher level of testosterone and so they typically  have more muscle mass than girls. Muscle requires more calories and energy than fat does, so boys need more energy than girls.

Activity Level
       Activity level is another factor that can affect caloric intake. Active people need more calories than non-active people do. For example an active teenaged boy needs around 900 more calories than a non-active teenaged boy and an active teenaged girl needs 750 more calories than a non-active girl does. The reason active people need more calories is because when they workout, they burn a lot of calories, so they need to refill them or they aren't going to be able to function well.


Age
       Age can also affect caloric intake. After age 25, a person's caloric needs start to drop around 2% every 10 years. The reason for this drop in caloric needs is because as someone grows older, they start to loose their muscle and gain fat instead. Since fat doesn't require as much calories as muscle, older people don't need as many calories as young people do.

Size
       The size of a person can change how much calories they need. Generally a person with a bigger muscle to fat ratio will need more calories because muscle needs more energy than fat, and a person with a smaller muscle to fat ratio needs less calories.

Sources:

http://www.powerbar.com/articles/183/eat-to-compete-in-high-school-sports-nutrition-for-teen-athletes.aspx

http://www.dummies.com/how-to/content/what-factors-affect-your-calorie-needs.html 
http://vaetiquette.com/x17220777.jpg