Most people, including me, until I was 44, thought of body fat as a single thing. You either had more of it or less. Diet and exercise change the amount. The belly fat was the part you could actually see.
The reality is different: there are two types of fat, and they are not the same problem.
Belly fat (subcutaneous fat) sits under the skin. You can see it, pinch it, watch it on the scale. It responds to caloric restriction and exercise, slowly but measurably. At 40+, it's frustrating and stubborn, but it's surface level.
Visceral fat is different in almost every respect. It accumulates inside the abdominal cavity, wrapped directly around the liver, the intestines, the kidneys. You cannot see it in the mirror. You cannot pinch it. The scale does not distinguish it from anything else. A person can lose visible belly fat and have visceral fat that hasn't moved at all.
After 40, visceral fat accumulation accelerates. Hormonal shifts, declining oestrogen and testosterone, rising cortisol sensitivity, change where and how the body stores fat. Chronic stress, disrupted sleep, years of low-grade inflammation all feed visceral accumulation preferentially. It is not a sign of discipline failure. It is a biological response to the conditions that 40+ life creates.