In a lifetime, we eat 50 million calories and 36,000 pounds of food. But on the average, we gain only 50 pounds. It's a very fine balance. Just a tiny imbalance can lead to weight gain. Food intake decreases with age, despite weight gain, which indicates more efficient conversion of calories to fat.
Metabolism is less efficient with age.
Also, as insulin goes up, calories are more easily stored as fat and are more difficult to burn. Weight increases on the average by one pound per year in adults until the mid to late 70's and then decreases somewhat as tissues atrophy, with considerable individual variability. It uncouples mitochondrial energy production, producing heat rather than fat. Leptin is one regulator of the process, but leptin is 5 times more effective in the young than the elderly. Like insulin, leptin sensitivity declines at the cell level with age.