I suspect Sarah wasn't really that old, if the story is true, that is. I tend to think the Bible is full of allegories and fables.
I read an article in Reader's Digest about two years ago, that had some excellent information on numbers and years in the Bible and how they are unreliable. One example was how many Israelites came out of Egypt...............impossible number considering the population of the earth at that time.............it would have been a very significant percentage of the entire population of the world, so not likely that the number was correct.
Another example was the number of men who prepared to enter Jericho. If there were that many, (100,000?) there would have been no reason to fear.............they could have just overpowered that city quite easily, since scholars estimate its population as being about 25,000 then.
So age was considered the same way. Maybe the 70 meant something else.