Question

How is growing older the same as an onion or rings inside a tree trunk or little wooden dolls that can fit inside another after another?

Answer 1

Each one shows its age. The rings inside of the tree are the years it has lived, same as our age growing yearly. The years add up, and don't stop until the death of the person/tree/onion.

Answer 2

as time goes on, more and more layers appear. in years, knowledge and expirience grows like concentric circles leading to more maturity and wiseness

Answer 3

The Figurative Language is a Simile