人們為什麼有不同的能量需要量

it is an easy question.
for different age of human, their activities are different.
for example, a child needs less energy than a man because
their activities need less energy than that of a man.
even for similar age, man doing different job also have
different energy requirement. for example,
a man working in a building site needs more energy
than a man working in an office. right?