Because of genetic factors, lack of physical activity, unhealthy eating patterns, or a combination of these factors. 
Obesity means having too much body fat. A child is obese if their weight is more than 20% higher than the ideal weight for a boy or girl of their age and height. Children become overweight and obese for a variety of reasons. The most common causes are genetic factors, lack of physical activity, unhealthy eating patterns, or a combination of these factors. Only in rare cases is being overweight caused by a medical condition such as a hormonal problem. Up to one out of every five children in the U. S. is overweight or obese, and this number is continuing to rise. Children have fewer weight-related health and medical problems than adults. However, overweight children are at high risk of becoming overweight adolescents and adults, placing them at risk of developing chronic diseases such as heart disease and diabetes later in life. Research has shown that obese children are more likely to be overweight or obese as adults. Data from the 2005-2006 NHANES survey show that in the United States, nearly 13% of adults age 20 and older have diabetes, but 40% of them have not been diagnosed. More than 2 in 3 adults are considered to be overweight or obese. More than 1 in 3 adults are considered to be obese. More than 1 in 20 adults are considered to have extreme obesity. About one-third of children and adolescents ages 6 to 19 are considered to be overweight or obese. More than 1 in 6 children and adolescents ages 6 to 19 Body mass index (BMI) is a measure used to determine childhood overweight and obesity. Overweight is defined as a BMI at or above the 85 th percentile and below the 95 th percentile for children and teens of the same age and sex. Childhood obesity is a complex health issue. It occurs when a child is well above the normal or healthy weight for his or her age and height. The main causes of excess weight in youth are similar to those in adults, including individual causes such as behavior and genetics.
http://www.med.umich.edu/yourchild/topics/obesity.htm http://www.webmd.com/children/guide/obesity-children http://www.webmd.com/children/guide/obesity-children http://www.nhlbi.nih.gov/health/educational/wecan/healthy-weight-basics/obesity.htm http://www.niddk.nih.gov/health-information/health-statistics/Pages/overweight-obesity-statistics.aspx http://www.cdc.gov/obesity/childhood/defining.html http://www.cdc.gov/obesity/childhood/causes.html
why do children get obese at such a young age