> Even if at 90 I had a body of a 20 year old, I would still have the mind of 90 year old. With all the experience, all the mistakes and presuppositions and biases.

I think you're underestimating how many of your thoughts, inclinations, moods and attitude are driven by the body (hormones, energy, etc). The mind is not separate from the body, they are one system. If you rejuvenate the body, the mind is also changed. I agree neuroplasticity is something that needs care as well, but it's simply not the case that it's a separate thing.