Oliver Sacks (in The Man Who Mistook His Wife for a Hat) also tells a great story of a blind patient with Cerebral Palsy that was so babied that she never acquired use of her hands. She had been carried everywhere and had everything done for her. She called her hands "useless lumps of dough". At 60 years old he and his team managed to get her to use her hands; she ended up going through all the infant stages of hand use. Despite being 60 and never having used them she was able to become a sculptor, feed herself, etc. He thought it might be a one-off but he ran into another patient with a similar situation (babied, unable to use his hands his entire life) who also acquired the ability to use them.

There is a reduction in neuroplasticity with age but there is more and more evidence that it is a smaller reduction than once though and a lot of the supposed reduction is due to attitude, expectations, and technique.

There have also been fairly recent studies with kids and adults born blind who receive surgery to correct the problem later on in life. Not being infants, they don't naturally learn how to build up a system of vision. It was long thought that acquiring vision after early childhood was impossible but that turns out not to be true either. It requires intense therapy to walk the person through the infant stages of learning: detecting shapes, matching colors, learning about edges, inside vs outside, recognizing people vs objects, learning faces, etc. Eventually reasonably decent vision can be achieved. We appear to be primed as infants to learn these things automatically so we never really think about how much work it is.

Yes. This is the crux of the book. That is, the brain can adapt and age isn't a factor, most of the time. An exception, for example, is language. It is more difficult for an adult to learn a new language. Not impossible, obviously. Just far more difficult because some parts of the brain are less plastic.

FWIW I've been reading it as a form of self-help book, sorta. That is, knowing how the brain works how best can I unlearn things. One of the takeaways so far is that to ditch an old unwanted habit you'd improve your odds if you replace it (and not just try to avoid it).

Also, the book emphasizes "use it or lose it." That's actually not entirely accurate. He gives numerous examples that are closer to "use it or it will be taken away." That is, brain cells (neurons) in proximity to the unused will slowly creep in and reclaim the unused for something else. Stop using your hand don't "lose it". You literally take away those neurons from yourself for something else.