It is true that with age we tend to have important physiological changes, but not everyone tends to be overweight and obese, however the majority does.
But obesity does not come alone, there are multiple factors that can cause weight gain and as we age we begin to have difficulty losing it.
Although it is different for men and women, we can generalize that there is a different mechanism between them.
The important thing here is to begin to understand what are the causes of obesity or what contributes to it.