What is the estimated percentage (in mass) of water in the human body? Is this percentage expected to be larger in the adult or in the old individual?
About 65% of the human individual mass is water.
The brain, for instance, has around 90% of water in mass, the muscles, 85%, and the bones have among 25% and 40% of water.
Younger adult individuals have proportionally more water in mass than older individuals.