I'm not an expert at all but have also thought about this quite a bit. Neuroplasticity is definitely reduced in adults, but so is free time and exposure to novel inputs. As we get older, a higher proportion of experiences are routine or at least things we've seen before, and we have far less free time. We are also increasingly behaving based on biases we accumulate and familiar patterns and just not using the parts of our brain we use for learning as much anymore. I often wonder if it's partly a kind of atrophy and whether you might be able to restore some neuroplasticity by "practicing." A related idea that occurs to me is that as you get older and learn about the world, you tend to trade curious openness for useful heuristics to some degree just to make life easier and more efficient.

The easiest example for me is learning guitar - I started when I was 5 and got pretty damn good over the years. It would be difficult to repeat that now in my 30s, but at least part of that difficulty would be because I can't really devote 3-4 hours a day nearly every day to it like I did as a bored kid out in the country. To be clear, I'm sure the differences in neuroplasticity come from both behavioral and neurological differences. It's just fun to think about how much is set in stone and how much could theoretically be "exercised" so to speak, and how much of the neurological changes are actually due in part to the behavioral changes as we age.