Kurzweil and Grossman put forth some good ideas, but the majority of what they've written isn't supported by scientific evidence. A lot of the book comes off as a thinly veiled shill for their health food company.
Consider the metaphor of maintaining a house. How long does a house last? The answer obviously depends on how well you take care of it. If you do nothing, the roof will spring a leak before long, water and the elements will invade, and eventually the house will disintegrate. But if you proactively take care of the structure, repair all damage, confront all dangers, and rebuild or renovate parts from time to time using new materials and technologies, the life of the house can essentially be extended without limit.