Crossword Genius

Being old is stressful with son turning 100 (9)

Ross

I believe the answer is:

existence

I'm a little stuck... Click here to teach me more about this clue!

'being' is the definition.
(I know that being can be written as existence)

'old is stressful with son turning 100' is the wordplay.
I cannot quite understand how this works, but
'old' could be 'ex' (prefix meaning former or onetime) and 'ex' is present in the answer.
'is' is found in the answer.
'100' could be 'c' (Roman numeral) and 'c' is present in the answer.
The remaining letters 'tene' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for existence that I've seen before include "A way of living" , "The business of being" , "State of being" , "survival" , "Way of life" .)

I've seen this clue in The Sunday Times and The Times.
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