Body size, age, gender and genes all play a role in the speed of your metabolism.
Muscle cells require more energy to maintain than fat cells, so people with more muscle than fat tend to have a faster metabolism.
As we get older, we tend to gain fat and lose muscle. This explains why your metabolism may slow down as you get older.
In general, men tend to have a faster metabolism because they have more muscle mass, heavier bones and less body fat than women.
Your metabolism may be partly determined by your genes, although this isn’t yet fully understood.
Genes definitely play a role in muscle size and your ability to grow muscles, both of which affect your metabolism.