> I wonder if people really would ossify after hundreds of years or eventually learn to be hyper-adaptable.

I'm kind of fond of Vernor Vinge's take in Rainbows End: people who undergo rejuvenation therapy go back to high school to learn a trade in the modern world. Part of the justification for this is if their brain is restored from a condition like severe dementia, they may have different personality traits and interests from their original selves, which will take time and learning to acclimate to.