Gradual (and not-so-gradual) weight gain over the years is not a function of age itself, but a result of the loss of muscle mass. People use age as an excuse to gradually stop moving, becoming more and more inert: they adopt a more sedentary lifestyle, so their muscles deteriorate, their metabolism decreases and they accumulate fat.
A sedentary life drags you into a dangerous cycle of greater and greater passivity, which makes you less motivated to move, and so frailer and more depressed. When you move, it feels more and more uncomfortable, so you are less likely to move, you gain weight, depression increases, and so on.
On the other side, movement creates a positive cycle since the more you move, the easier it is moving. It preserves your mood, muscle mass and therefore your metabolism. Your body stays leaner, your joints healthy and mobile, you feel more positive and motivated. When you move it feels good and you want to move more.
In their fascinating study, Wroblewski’s group (link: https://www.ncbi.nlm.nih.gov/pubmed/22030953 ) show the MRI scan of quadriceps of a 40-year-old triathlete, and then they compare the MRI scan of quadriceps of two 70 years old men, one sedentary, the other one a competitive triathlete, looking at the fat/muscle composition:
Considering the strong similarity between the muscles of the youngest and the oldest triathletes, it suggests that it is possible to preserve muscle mass and strength if we stay active through our life.
So never stop moving!