Once you hit age 70, you can throw BMI numbers out the door. Not that you shouldn’t care about it at all. Researchers have found that waist to hip ratios are a better determinant of whether you have a higher chance of premature death.
This would certainly be applicable to younger individuals also. When you waist is growing at an alarmingly higher rate than your hips, you have a problem. You don’t need a rocket scientist to tell you what’s black and white.
Learn to regulate your body to adhere to the law of averages instead of being the exception. If your tummy is bulging, eat less and exercise more. If you are all sticks and bones, then put some food into that body of yours.
A better indicator: If all of a sudden you don’t fit into your clothes due to some extreme weight gain, then you are more likely to face a premature death.