Well personally, I don't see 12 as 'old' - but then my own lad is over 21 so I wouldn't, would I? 
However you come at it, I think he's getting on a bit though - and if he didn't have such a good life before he came to you?..........
See what the vet says.
It sounds as if you need to have his teeth properly taken care of and it might well be better to have it done now and give him some extra good years rather than leave him to soldier on to a point where he really was too frail for a GA.
(My own old lad had a full dental at 16 and while previous vets had demurred due - mainly - to their teams and facilities, his current practice expressed a fair confidence that they could get him through it. As it was, his teeth were so horrible that I had little choice and he came through just fine with some good fluid support etc - and hasn't looked back, even with most of his remaining teeth now gone from his mouth.
)
I'm afraid that it's not a massively cheap procedure though. Sorry to mention such practicalities but I wouldn't want you having conniptions on the practice floor. 
Good luck, whatever you decide for him.