The average weight for a given age differs with time, as obesity rates have been steadily increasing with time, as reported by ABC News; however, the World Health Organization, Gerontology Research Center and the United States Centers for Disease Control and Prevention has collected an abundance of data to determine the current average weight for each given age and gender while still taking into account societal and cultural influences. The average weight for a given age can be measured using three different techniques: head circumferences, body mass index, or BMI, and waist circumference. Most health institutes are concerned with whether the average weight for a given age is healthy given the overall spike in obesity rates.
Head circumference is typically only used to measure children who are up to 36 months old. It does not provide a good measure of body weight, but can provide insight regarding to whether the children is growing at a healthy rate. The waist circumference, on the other hand, basically measures the additional fat that is carried on each individual.
The BMI is perhaps the most commonly used to determine whether the given weight is healthy based on the height of the individual and the age. Those people with a BMI of less than 18.5 are considered to be underweight while those with a BMI between 18.5 and 24.9 are considered healthy. Those people with a BMI between 25.0 and 29.9 are overweight and those with a BMI of more than 30.0 are considered to be obese.