Your dog at 6 years old would be considered middle aged, basically in his prime, most of the growths that occur around the face are benign, Maya had one on her eyelid and it got a bit bigger than disappeared on its own sort of grayish coloured, forgot the type it is called, but common in young dogs. As far as anethesia in recent years pretty well vets have changed over to newer safer gases like propenol (sp) and isoflurane which are human grade, it is pretty rare now to have complications unless the dog has reaction like hyperthermia and since yours has already been under before without problems that won't be an issue, other problems could be blood clotting disorders but bloodwork is usually done just prior to make sure that is not the case, I know of 15 and 16 year old greyhounds who have had dentals done and came out just fine, so age is not really a factor, once my dogs turned 7 I asked for senior blood panels and physicals to be done prior to any surgery to see if there are any health problems that could be a concern, it does not mean if their is a health problem that surgery cannot be done, but it helps the vet know whether extra monitoring needs to be done or different choices of drugs should be used to induce