When are you grown up? The short answer is Never.

Auntie Nan, so the story goes, asked on her sixty fifth birthday: "Am I grown up yet?"  It's a good question.  Forty years later, I have the answer.
"No."
The fuel ages.
The fire does not.

The answer has competition.
- "Yes, if you have obtained the age of majority."
- "You are only as old as you feel".
- "Yes, but you will be raised from the dust of death to judgement at a later date".
- "It's Groundhog Day until you learn something useful.  That's karma"

The evidence of aging is plentiful.   As for growing, babies stop just over five feet tall and develop wrinkles with liver spots.  As for being an adult, the root meaning is "ripe" and is answered as soon as you can reproduce successfully.

My viewpoint:
The fuel ages.
The fire does not.
It's a process.

In English, the fuel is named  "I".
We don't know the nature of fire.
It's okay not to know the answers
but a good question helps.