I was surprised to learn that having a slow metabolism is actually rare. It's not uncommon to blame metabolism for weight gain but since it is a rare ocurrance, weight gain is still likely caused by eating too many calories and not burning enough of them through physical activity.
Metabolism does slow as we get older so eating less calories or increasing physical activity is probably a wise thing to do as we age. For the most part, the more lean muscle we have, the more calories we'll burn at rest. The amount of calories we burn at rest is known as our BMR (Basal Metabolic Rate.)
This is why strength training is so highly recommended in terms of losing weight. Our muscles burn calories even after we are done working out and muscle burns more calories than fat. Strength training burns fat and builds muscle which is what helps to boost metabolism.
Another reason for a slow metabolism is dehydration. When we are dehydrated our body temperature can drop slightly and this causes our bodies to store fat in order to increase the temperature. This is another reason to drink more water.
Having a slow metabolism is rare so you have a good chance of boosting it by exercising and watching your caloric intake.