Hey Rufus!
I've given this a lot of thought recently because my 25 year-old daughter, who never had to give a thought to how much or what she eats and was always on the slim side, has recently found herself overweight. She was a dancer, a college athlete, and very active. She's less active now. I never wanted her to get into the body-image issues so prevalent with young women, so I tried to always just stress eating healthy, wholesome food. So she's having to learn to negotiate the food thing.
I think most of us who were normal weight through our teens and early 20's just kept on eating in the same way while our bodies and our activity levels(possibly) changed. Although our bodies required fewer calories, we just kept eating the same, and little by little, our weight crept up. So, it only makes sense that if we keep on eating as we did in our 20's, we find ourselves "in deep doo-doo," to quote a former US president.
So for those of us in our 50's and 60's it's a whole new world. I agree with the other posters. Look at what you've been eating and dial it back a little. Instead of a whole pub sandwich, just eat a half with half as much beer. Get a little more active. You don't have to throw out the things you love--just make'em a tad smaller. And continue having fun!