Researchers has found that between the ages of 20 and 60 metabolism slows down by about a third. Human aging begins in late 20's tending to loose muscles and collagen while gaining in fat. The largest component of your metabolism is your basic metabolic rate, which is the amount of energy your body uses just to maintain functioning at rest. Its surprisingly accounts 50-70% of the energy used each day. It is said that your BMR decreases at 1-2% per decade after you reach the age of 20 and up to 10% per decade after the age of 45.