It does get harder to keep the weight off as a person gets older. I've posted about this before, but I just completed a Developmental Psych class, and the reality is that in middle age, individuals lose height. These are facts from the text: from 30-50 yrs, men lose about 1/2" in weight and another 3/4 inch from 50-70. Women can lose 2" from 25-75yo (due to loss in the vertebrae). In 40s and0s, skin begins to sag because of loss of fat and collagen in underlying tissues. After age 50, muscle loss occurs at a rate of 1-2% per year. There is a loss of strength in the back and legs. Maximum bone density occurs in mid-late thirties and then begins to be lost, accelerating in the 50s. Women's rate of bone loss is twice that of men ... these are natural, biological body changes. As the text suggests, exercising and eating healthily helps A LOT, especially weight training for women.
I have staved off osteoporosis not by taking a boatload of calcium, but by stepping up my weight resistance. But even now, for me, my body is very sensitive. I have developed some sensitivities to food that I did not have in my 20s or 30s. Because I recently tried to reintroduce one of those foods into my diet, my gut became inflamed and I gained *3lbs* just for no reason - other than inflammation. It will fall off again (I am determined!) but many people have sensitivities/allergies of which they are unaware. These can play havoc with the body weight.
I really don't get bent out of shape about how a body looks. Some larger people are often healthier than skinny people. It depends on a lot of things. If the skinny person is starving to stay thin, they are depleting their body of many things that the body needs. OTOH, the larger person may be eating very healthily but just a tad too much. A person needs to judge their health on annual check-ups, including a blood panel. When my doctor tells me that my cholesterol is excellent, my BP is excellent and I have no pre-diabetes (for which I am not at risk anyway), then I am quite happy. Not so happy when I become bloated because I ate the wrong thing, gah, but I'll get it out of my system and all will be well.
My dh, btw, is overweight. I worry because he also smokes, but no-one can force another to be healthy. These are his choices. I don't love him any less.