Age alone should not decide it: healthy older dogs that reach surgery in reasonable condition survive GDV at rates not dramatically worse than younger dogs, and go on to normal lives. What legitimately tips the decision is what age often brings with it, heart disease, other illness, poor recent quality of life, and what the team finds on the table, because dying stomach wall or spleen worsens the odds meaningfully in any dog.
The two questions that cut through at 2am: first, what was this dog's quality of life yesterday, honestly; a dog that was thriving has a claim on the fight that a dog already failing did not. Second, ask the surgical team for the decision tree inside the surgery, at what findings would they recommend not waking the dog up. Agreeing that in advance is what lets you consent with a clear head.
GDV is one decision where there is rarely time for an outside second opinion beforehand. Where we can genuinely help is afterwards: reviewing the records of an episode, whatever the outcome, so you understand what happened and what was, and was not, possible.
Send what your vet said and quoted. A veterinary surgeon reads it and comes back to you with what they can do and what it would cost, before you pay anything. A written second opinion is from $250.
Send the case, hear back from a surgeon with what they can do and what it costs, and only pay if you go ahead.