Esspee I profoundly disagree. Recent research has shown that the relationship between food, calorie values and how our metabolisms work is far more complex than you suggest and the simple equations of the past calories in + calories used + fat, no longer holds good. How our very different metabolisms handle food and deal with different forms of calories, plus the constituents of our biome means that our bodies handle food in different ways.
Recent research is also discovering genetic elements that affect how peoples bodies handle food and, while, it has long been recognised that there is a genetic component in extreme eating and weight problems like Praader-Willi syndrome, there are other genetic changes that affect how people metabolise their food.
I put on weight after the menopause, despite reducing how much I ate drasticaly, I also was retired early(redundancy) and bought a big old house to restore with a big overgrown garden, where iwould spend 4 or 5 hours a day clearing and digging, when not undertaking house restoration work.
Nothing stopped the weight gain, until I hit a plateau, where no matter how much I ate or didn't eat, my weight never varied, Then when I was about 70 Michael Mosely introduced the 5:2 diet. I followed it, but limited my calories on non diet days, as far as possible to an average of 1200 calories which obviously excluded, holidays, birthdays and special celebrations. I lost 2 stone in 4 months without any effort and eating more than I had eaten in years. Whether it was the diet, or just that my hormones had adjusted with age. I do not know. Again since then myweight has been stable regardless of days or weeks of indulgence or dietary austerty.
I have just clicked on the link in razzaatazz's post and totally agree with it, especially the link within the link to oestregon and weight gain.
Another cause of fat gain, if not weight gain, is that as we get older we shrink in height. The cartilege disks between our spinal bones shrink and become thinner and we can slowly lose as much as 2 inches in height by the time we are 80, if you have osteoporosis or a baxk injury that leads to a crushed vertibrae, it can be even more and requires even further weight loss to stay within approved parameters.