Assuming you mean this: https://en.wikipedia.org/wiki/Methuselah_(tree)

It says that its trunk is about 5,000 y.o. as counted through growth rings. I would think most people would understand this to be the proper age of a "tree".

You are 10 years old, I am also 10 years old. We are all 10 years old. [10]

[10] https://www.nytimes.com/2005/08/02/science/your-body-is-youn...

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...