I have known so many girls and guys who were in very good shape in early 20's and then once their metabolism slowed down, they got fat.
You can't stuff your face and load up with fuel and not burn it. It only gets stored as fat. People don't understand, you only need to eat once a day if you going to sit around and not work out. Drinking adds a lot to the equation. I can't tell you how many of my friends from growing up continued to drink like in college and got beer fat. Now they are so out of shape and fat - they just give up. It's too much of a mountain to climb to diet, workout, and do cardio - it's just easier to accept the fact that they are fat and stay that way.