I’m pretty sure all the water on this planet is the same age.
No, it isn’t. Water molecules form and break apart all the time. An example would be burning gasoline: the hydrogen from hydrocarbons combines with oxygen in the air to form brand-new water.
“I’m pretty sure all the water on this planet is the same age.”
Some of the water on the Earth is a younger age, because the water was synthesized by biochemical reactions after the formation of the Earth.
Ah... but some water has been through kidneys, and some has not... :)