Hey there! The answer to your question is:
Age
Here's why!
The volume of skeletal musculature drops and the percentage of fat tissue raises with age. It is noted that the reduction in muscle mass related to the total body may be entirely accountable for the age-related decreases in basal metabolic rate. The amount is minuscule and the only reason it does slow down is that you manage to lose a tiny bit of muscle mass as you age. As long as you resume lifting weights and eat right, your metabolism will be just fine.