Our basal metabolic rate (the rate at which we burn up calories when doing no physical activity at all) does very gradually begin to slow down over the years once we reach the age of thirty or so. It is estimated that for every five years older than 30 we are, we need to consume about 50 calories a day less in order to maintain the weight we were then. That means by the age of 60, if you haven’t reduced your daily calorie intake to about 300 calories a day less than at 30, you will have slowly put on weight. (After sixty, body fat percentage tends to slowly decrease again, naturally.)Part of the reason for this slow-down is that we lose lean tissue mass (muscle) as we age. Muscle is more metabolically active than fat and other body tissue. Another reason is that we tend to use up less energy in activity as the years go by. There is also some natural slowing down through the aging process itself.
This only way to stop or minimize this slowing down of the metabolism is to increase the amount of exercise that you do, and include toning/strengthening exercise to keep muscle mass. It can be done – but, it seems, few manage it.
For your health’s sake, a few pounds or even a stone on your slim early adulthood weight won’t harm you, as long as you eat healthily and stay as fit (although plenty of new research indicates that calorie restriction is a key to longer life). Trying to maintain the very slim weight of your youth may be an unrealistic target.