It is an equal opportunity killer. Nobody is immune to it.
Obesity is the underlying cause of a majority of diseases affecting mankind today.
It is this continued intake of the same amount of calories in the face of progressive slowing of metabolism due to aging that leads to weight gain. A rare cause of weight gain is Cushing's syndrome, which also causes a multitude of other symptoms, such a irregular menses, excessive hair growth, fatigue, diabetes and hypertension. Hypothalamic obesity is another rare cause of obesity. An endocrinologist can appropriately evaluate for the endocrine causes of obesity. There are many different reasons in different individuals.