On a scale of 0-6, I would give this essay a score of 2 or 3, which corresponds to an essay that is seriously limited and maybe somewhat flawed.
In order to score in the upper half of the scale (4-6), you need to have a solid structure down. To have a solid structure, you need an introduction that states (a) whether or not you think the argument is flawed, and (b) your explicit reasons for doing so. From the first paragraph in your essay, I am actually not entirely sure what stance you are taking, or why you are taking it. The next two or three paragraphs can be devoted to supporting your reasons for your side, and the last paragraph can conclude your essay.
You made a good point in identifying that improvements in medical technology need to be balanced by the costs. This is important to mention, but it's not the most serious flaw in the argument. Let's say costs weren't an issue - would the argument still be flawed? (Yes). The reason is that medical technology does other things than prolong life, and even if it were to continue, we would not know whether it would prolong life beyond 90 years; nothing tells us that it will do that.
A second flaw in this argument is that he says that "each generation of Americans" lives longer than the last, but in his conclusion he states that "most children born in America" will have longer life expectancies. He's referring to two different groups of people here. This is a pretty subtle one, but I'm certain you'd get bonus points for mentioning it. It could very well be that being born in Europe would give you a longer life expectancy, not necessarily due to medical technology, or something like that.
Think carefully about the flaws in the essay prompt - there are several. Before you even begin writing, list 3 flaws that you feel comfortable talking about, and devote a whole paragraph to each one.
Your last paragraph about Nostradamus, while interesting, does not really add to your argument. How does the evidence shown there help prove your point about life expectancy and medical technology? Ask yourself that every time you want to write something. If you can't, then your evidence doesn't belong in the essay.
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