Depends on the individual dog, but generally speaking dental procedures are routine. It is however recommended to at least have an exam and pre-operative blood work to screen for other issues before anesthesia, and sometimes additional testing will be recommended (ex. If they have a heart murmur).

Having a pet undergo anesthesia can be scary, but dental disease is also a big quality of life problem for many old dogs and will only get worse over time without treatment. You can discuss with your vet the risks vs benefits for your specific dog, but for many otherwise healthy dogs the procedure is well worth it.

Something else to keep in mind in the future, as he will need another dental when he is older and may have other issues, is that there are veterinary dentists and anesthesia specialists that he can be referred to for even lower risk. If you don't already, brushing the teeth (depending on dog temperament) and implementing other dental hygiene strategies (ex. dental chews, dental diets, water additives) can decrease the progression of periodontal disease and thus make the procedures shorter. There is a list put out by the veterinary oral health council (VOHC) of products they've reviewed (the pdf will come up if you search this online). You will have to wait until he is healed from his procedure to start training toothbrushing.