> wonder how I can avoid that and brain plasticity excuses
I've worked on this question personally. I decided that the obvious first step to gaining the mental flexibility of youth, is to mimic youth:
* Explore: When we're young, we constantly try new things, even when there isn't an apparent ROI. We try new arts, new experiences, new ideas, new hobbies, etc. We are not afraid to ignore the established way and invest in something new - often for the novelty (or rebellion) of it. We give the new things time; we play. We are curious, not critical - we wonder why and explore the idea instead of criticizing it and shutting it down. When we are old, we often stick to what we know well and criticize the rest.
* Push yourself: In school or as a junior employee, you can't say 'I've always done it this way' or 'I'm not interested in learning something new'. You have to learn and adapt. When we're old, power corrupts - most people make those excuses and they are generally accepted. Nobody else will push you, as a rule.
There are limits in life; I don't have as much free time now as when I was young, but that's not a deal-breaker (and I use time much more efficiently now, including by prioritizing and by knowing myself much better). Also, I don't 'play' like a 6 year old or even a 25 year old; I do it my way.
I also saw it as an interesting experiment: How much of mental changes were due to changes in practice and how much due to biology. I can't provide empirical data but especially Exploration seems to have changed my life, not only mentally but significantly, emotionally: I'm much more optimistic, less jaded, and more emotionally connected than I was. Life is invigorating. A warning though: I'm challenging some norms of age and therefore peers don't grasp and sometimes reject me. I wish I could get through to them.