First of all, keep in mind this is my opinion, if you feel differently, I am fine with that, we can all still be friends...
Many people feel that you can radically change your metabolism by the foods you eat and how and when you eat them, or using some magic pills or supplements. People also say that if you replace 10 pounds of fat with 10 pounds of muscle that your metabolism is going to shoot through the roof, the truth of the matter is that fat burns 2 calories per pound of fat per day while muscle burns a whopping 6 calories per pound of muscle per day so if you replace 10 pounds of fat with 10 pounds of muscle you are burning an incredible 40 extra calories per day, so if you drink your coffee with sugar and no cream then you can have an extra one with no ill effects ;)
The reality is, the work involved in achieving the new 10 pounds of muscle is what is burning the calories the most. So really any type of activity that burns calories is all that is needed, of course combine this with healthy and moderate eating and you will not have a problem with your metabolism, because the problem is not your metabolism, the problem is lack of exercise and poor eating habits.
In general your metabolism slows down about 5% every decade and the main reason for this is because over time you do less and require less muscle so you tend to have less lean body mass (more fat and less muscle).
So if your base metabolism was 1600 calories at rest in your 20's, you would burn 1520 calories in your 30's, 1444 calories in your 40's, 1372 calories in your 50's and 1303 calories in your 60's. That is of course if you did nothing to maintain your lean body mass. So we are looking at a metabolism loss of about 300 calories from your 20's to your 60's. Do you really think that's the reason why people are having trouble with their weight?
All you would need to do to offset that amount of calories at the age of 60+ is go for a brisk 20 minute walk each day, or drink water instead of juice. Why not do both and really get into shape!
All you really need to do is make small everyday lifestyle choices that include some exercise each day and healthy eating habits.
Please stop blaming your metabolism and take responsibility for what the real reason is for the extra 10-20 or more pounds you know you don't need. It's never too late to do something about it, the first step is to stop making excuses and start making plans towards a healthier lifestyle!
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