Generally, older children (school age and teens) are at greatest risk for becoming obese. According to the latest National Health and Nutrition Examination Survey (NHANES:2007-2008) the prevalence of obesity among US children and adolescence is as follows:

NHANES  DATA

When we compare these rates to childhood obesity rates in 1980 we can see that the rates of pediatric obesity has tripled in the last 30 years.

AGE	2007-2008	1980
All (2-19 year olds)	16.9% obese	not available
2-5 year olds	10.4% obese	not available
6-11 year olds	19.6% obese	6.5% obese
12-19 year olds	18.1% obese	5.9% obese