The river is a thousand years old. The oldest water in it just came from the spring a few days ago.

Yet take away any water younger than a week, and the river is gone.

How do you measure an age of a process defined by change?

While its true that most cells in our body replace once about seven years, humans do have e.g. neuron cells that are about as old as the individual: https://www.snopes.com/fact-check/does-body-replace-itself-s...