Because of different genetics, age, lifestyles, and environment, our bodies have certain differences in their chemical composition between individuals.
The easiest example to explain this is the following.
Muscle tissue contain less average water, than fatty tissue.
Therefore, if you're an athletic person,
you will have lower average water composition of your body than,
if you were sitting on you ass and eat cheeseburgers all day building up that gut.
Old people, on average have less soft tissues per kilo of their body weight than a young persons, therefore, they also have lower water composition of their body.
Some people have thicker bones, are shorter, or taller than the average, all that accounts into the percentage of your body that is water.
It's all in the genes. How the human body is planned out from birth to end of life. The water percentages in the human body vary based on their age. The younger one is the more percentage of body water they have. This makes infant more prone to dehydration compared to children which has less water composition and more less in adults. The difference in percentage is based out on your body's metabolic needs for sustenance and growth.
As you grow, body water composition decreases to about 60-65% to accommodate other organic materials like bones, tissues, and inorganic substances. It's not that you body has decreased the capacity to hold water, it's more like your body accumulates more organic substances that decrease water's proportion to the entire composition as a whole.