That ignores a huge psychological aspect of weight gain, alongside several questions that contribute:

• How does the body manage when someone goes from incredible, disciplined health - to no care
• How do you maintain a calorific deficit in ill health, or with limited mobility?
• How do you stop someone eating for fun when they're in their twilight years and they want to enjoy themselves?
• How do you motivate someone that has gone from good mobility to bad mobility?
• How do you navigate a caloric deficit safely with both advanced age, retirement, AND diabetes?

I can say that for the latter points, it is incredibly hard to do so, looking at my parents and with people I've trained with. Furthermore, going from fighting fit to limited care can make you explode in both weight and joint issues. I've trained with a few army guys or people with pro sports experience, and it's mad how even in your forties you can go from a sub-twenty minute 5k to throwing up on a mat during a 5 min spar.