It isn't quite as easy as you might indicate.
I'm 40 and eat a better balanced diet than when I was 20. I exercise, but weight has gradually increased over time. I was at the bottom end of normal for what BMI charts say I should have been @ age 20. I am now about 15 lbs into the "overweight". My doc says I am fine because I have more muscle, but he wants me to hold the line.
I made some changes to exercise, working out 5 times a week in the morning and cutting out all soft drinks and after dinner snacking. I dropped 5 lbs in two weeks. i was hydrating a lot so it wasn't water that caused the drop.
After two weeks, same diet same exercise I dropped 5 more pounds in two weeks. I was feeling great. I was hoping for another 10. But guess what? Two months later, same diet same exercise I didn't drop a single pound. I am not sure how to explain it. It is like my body reached a certain point and compensated for the caloric drop by going into a lower metabolism rate.
When I was 20 I couldn't gain weight no matter what. Now, I know that 160# is a place that my body just doesn't want to drop below. I understand that I could increase exercise more or cut out even more food... but is it worth it?
I am convinced that BMI might be a guideline, but it isn't gospel. I can still run a mile at a good clip and keep up with the kids. What am I gaining by dropping into a somewhat arbitrary scale if I am healthy already?