Is generally acknowledged that as human beings age, it becomes more difficult for them to lose weight, or even maintain their current weight. People find they have to eat less and/or exercise more to maintain a healthy weight. Which is the most likely explanation of why weight loss/maintenance becomes more difficult as people age?

As humans age, they becomes less active and use their muscles less; therefore, less chemical energy is converted to thermal energy.
As humans age, the process of extracting energy from the food becomes less efficient, so more food is needed to provide the same amount of energy.
As humans age, they become injured more often and need to increase the amount of food they eat to make repairs to body tissue.
As humans age, the body does not convert chemical to thermal energy, so they do no burn off any calories when exercising.