Dr. Michael DeBakey, one of the founders of open-heart surgery and thoracic aortic aneurysm repair, developed a thoracic aortic aneursym in his late 90's and underwent surgery for it at the age of, I believe, 97. According to media accounts, the operation went well, and he made a good recovery. Dr. DeBakey died, I think it was a couple of years later, at maybe age 99, of (as far as I know) unrelated causes. I might have his exact age wrong, but he benefited from the surgery and died later on of, basically, old age. This happened within the past ten years or so, and the media reports can easily be accessed on the internet, if you're interested.
As a patient, I've never heard of an age 70 cut-off for thoracic aortic aneursym repair, at all. Lots and lots of people are still healthy enough to undergo open-heart surgery in their 70's, and a few still are in their 80's. Think of all the people who have coronary artery bypass surgery in their 70's. It's mainly the OHS aspect of it that you have to be able to tolerate and recover from. The rest of it, as far as what has to be done inside your chest, goes to the skill of the surgeon. So if you're in your 70's, and you have a thoracic aortic aneurysm that needs to be repaired, and you're otherwise healthy, and the surgeon that you are talking to thinks you are too high-risk because of your age alone, then I say find a better surgeon.
Probably you would have to be Dr. Michael DeBakey to get a surgeon to consider repairing a thoracic aortic aneurysm on you when you were in your late 90's, but it was done for him at least, and so clearly it is possible with the right patient, the right surgeon, and the right team.
You are right that typically an aneurysm is repaired once it reaches 5.5cm or if it is growing greater than 0,.5cm per year. However, the comment about not fixing the anuerysm in a person over 70 years old is not a hard and fast rule. It depends on the patient, and the surgeon. If you are a relatively healthy person, and not debilitated by chronic disease, a surgeon is more likely to operate on an older person.
Bottom line, there is nothing special about using the age 70 as a cutoff. The decision is made on a case to case basis.