How long would you like to live?
I think any answer that's not "as long as possible" is motivated by wanting to maximize health. It's difficult to want to live past your 80s when you saw your grandfather slowly deteriorate over the last decade of his life. It's difficult to want the last glimmer of life left in a body constantly in pain and incapable of properly walking, breathing, or thinking.
But what if you could keep your youth reasonably intact over the entire course of your life? What if everything about your life right now was the same but your birth certificate said you were a hundred years older?
At the cellular level there's nothing obvious that shows we wouldn't theoretically be able to do this. Once humans figure out the gene therapy to lengthen telomeres and repair the histones of the epigenome of each of the cells of your body, your biological age can be effectivly reset with no information lost.
As far as David Sinclair can tell this means you'd be able to live a healthy life for as long as you want.
There are some interesting questions here about whether life would continue to have meaning if you knew it could be extended forever or whether the desire for a longer life isn't just the flailings of a power-hungry ego, but as far as I'm concerned more time to chew on these questions is better. I'd certainly not choose to artifically shorten my life to give it more meaning so it seems foolish that extending it would take away meaning.
The meat of the book for me is about what I can be doing before we have full gene therapy to be extending my life. Taking Metformin, NMN, and resveratrol seem to be the only significant alterations to my current lifestyle that might have a strong positive effect.
Other than that his advice