I'm 13, 5ft 9 and I weigh 135 lbs. Am I fat?

You are not fat yet. You'll be fat if you cross the level of weight is 143.3 according your height 5 feet 9 inches. 

According to medical charts, you're not overweight, however it's difficult to determine whether someone is overweight without being able to see them. People have different body shapes, and muscle weighs more than fat, so if you're athletic that could be something to bear in mind.

In fact there are many factors surrounding healthy and unhealthy weights, and the numbers don't always tell the full story. Let me explain a little further:

• Healthy and unhealthy weights need to take into consideration whether the person is male or female.
• Athletic individuals tend to weigh more due to having more muscle than the average person.  Take body builders, for example, who would be considered obese by medical charts. 
  
• Younger people tend to fluctuate in weight as they're growing.  A lot of overweight children will lean out as they grow taller.

Having said all of that, it is important to maintain a healthy lifestyle.  The best way to ensure that you're on the right track is to get plenty of exercise and eat a balanced diet that's rich in vitamins and minerals.