What's the oldest age at which a grandmaster played their first rated game? Of all grandmasters past and present, what is the oldest age at which any of them started playing chess?
Define started playing chess:

The day that person played their first (FIDE, or equivalent) rated game
This isn't an answer to the exact question posed, but it may nonetheless help.
Lajos Portisch learnt to play aged 12.
Akiba Rubinstein learnt at 14 or 16. (Sources vary.)
Harry Pillsbury learnt at 16.