We have an 19 year old cat who comes in for routine teeth cleanings at the clinic. They put a catheter with IV fluids on him, and put him under, clean his teeth and bring him back up as soon as possible. We do a pre injection, and then use gas[so the gasing isn't so scarey].
If the blood work doesn't come back okay, your vet won't do it. Everyone can tell you any horror story in the world, however the circumstances for each animal is different, and you're only hearing one side of the story. Anything can happen to an animal of any age going under anesthesia, so you do need to be prepared, but not overly worried, if the vet thinks your cat will do fine, your cat will probably do fine.