Unfortunately, it's not that simple. Most diets and weight loss products are useless for permanent weight control and a few are downright dangerous. There are also many overweight children.
Men often start to gain excess fat in their late twenties and thirties.
Many do not consider they are too fat, even though their excess weight is a health hazard. The number of overweight women rises steeply after age 40. In younger women, there are more women who are underweight than overweight. Many young women see themselves as fat even when they aren't and this leads to poor diets and eating disorders. It is only upper body fat that is a health hazard.