While it is true that your metabolism does slow down with age, there are some things you can do to increase you metabolism. Putting on just 5 or 10 lbs of lean muscle mass will rev up your metablolism, and cause you to burn fat while resting.
Eat oftern, (every 2 or 3 hours) But healthy. Get plenty of healthy protien, and fruits and vegetables. Increasing your intake of high fiber foods is one ot the best ways to increase your metabolism, as it takes more energy to "try" to digest the high-fiber foods. Plus fruits and veggies are low in calorie and high in nutrition.
Get plenty of rest. Lack of sleep can decrease the number of calories you body burns just resting.
Last but not least, omega 3's gives a powerful boost to your metabolism.Fish oil increases levels of fat burning enzymes and decreases levels of fat storing enzymes in your body. Make sure to buy pharmaceutical grade having 540mg EPA and 300 mg DHA.