Metabolism and Age - Are they related? Yes and no.
People think that because they age they automatically have a slower metabolism than when they were younger. Yes, your metabolism does slow as you get older, but not for the reasons most people believe. It does not just slow because you get older, but instead because you stop moving around as much.
Your metabolism works off two main things: genetics and lean muscle mass. If your parents were lethargic/athletic/ping-pong pros, you will be more disposed to lead that kind of life based upon your genetics. Not all people are that way though. The main reason metabolism slows with age is based upon your lean body mass. When you're younger you move more, and as people get older they move less. Therefore the lean body mass of an individual will decrease unless they continue exercising or begin to exercise even more. Also, if you have very little lean muscle mass then you will not burn as many calories in a day as someone who has lots of muscle mass.
In closing, if you want to make sure your metabolism doesn't slow as you begin to age then exercise and move to stay in shape, keeping that lean muscle mass in check.