Anything that is in the world when you’re born is normal and ordinary and is just a natural part of the way the world works. Anything that’s invented between when you’re fifteen and thirty-five is new and exciting and revolutionary and you can probably get a career in it. Anything invented after you’re thirty-five is against the natural order of things. — Douglas Adams, The Salmon of DoubtI'm getting mighty close to the third phase where Adams claims that your mind closes off and rejects new things. I can see how it could be true, and how it could happen. By thirty-five you're probably getting settled into a career. You've gained a fair amount of experience, and your work life has become more predictable. You've learned a good number of tools and processes to do your job, and your brain is starting to feel a wee bit full.
Besides, there are so many other things pulling for your attention - a wife, kids, a house, the latest episode of Game of Thrones... There seems to be little time to learn the latest programming language or development process. What you already know is good enough, right? It's served you well up to this point, and now you should be able to coast until you get into a management position or maybe hold on long enough for retirement.
I don't want to be that guy. I will resist that future me at all costs, and there's a good way to fight that tendency of becoming complacent. Learn something completely and utterly different from what you are doing now. This does not mean that if you are a programmer you should learn some graphic design (although you should; it will help you better understand designers), or if you are a web designer you should learn how to program (although you should for the same reason).
Get aware from software. There's a much bigger world out there. You could pick up a musical instrument, learn a new sport, or practice a new art form. Study a subject that you have little exposure to, whether that be economics, history, literature, or some field of science. Find something you're interested in that you have never done before and seriously learn it.
Why? Because it will be hard. Learning something new is hard, and if you've forgotten what it's like, it will be good to remember what it feels like to have to put concerted effort into something to produce only the most basic of results. It will twist your thinking and expand your mind so that there's room for new ideas to take root. You'll come back to your day job with fresh ways of thinking and a new appreciation for what you already know.
The difficulty of learning something completely new will make learning that new language or framework that you need to know seem like a walk in the park in comparison. When learning a new tool or method for a skill that you've developed for years, you have plenty of ways to connect that new knowledge with what you already know, and it's relatively easy to expand your knowledge in familiar areas. But if it's something you've never done before in your life, it's much harder to relate it to what you already know, and you have to work much harder to get good at it.
When learning this new thing that you've decided to pursue, bring some intensity. Don't just dabble in it here or there. Devote some real time to it. If you are thinking about it everyday and putting in concentrated effort multiple times a week, you'll start to see another benefit to this exercise. After six months or so you'll begin to appreciate the depth of this new thing you're learning and how incredibly little you know about it. You'll be improving rapidly, but at the same time you'll increasingly become aware of how much more there is to know - advanced techniques, development of style, and the rich history that surrounds it.
Recognizing what you don't know is a valuable skill, and re-experiencing what it's like to be a newbie is a good way to develop that skill. We think we know so much about aspects of life that we've never really done, only observed. Economics, climate science, and teaching come readily to mind. Do you think you know what it takes to do these things? Do you think it would be easy? Try it, wholeheartedly. I encourage you. You'll likely find that it's not as simple as you thought; that it takes a lot more work and effort than you realized. You'll likely come out of the experience with a much greater appreciation for those that do it for a living and do it well.
You may even come away with a new perspective on your day job. Are there things that you do everyday that could be done differently? Better? What things don't you know about your area of expertise, and what things do you think you know that just ain't so? Learning something entirely new will help you reevaluate what you already know in a new light and notice where the gaps are.
Finally, learning new things will keep your mind supple and flexible. You'll be actively resisting that tendency to become rigid and static. I remember reading about a 100-year-old woman who was lamenting the fact that she didn't start learning the violin at sixty. If she had known she would live that long, she would have started, and she could have been playing the violin for forty years! Learn from her regrets. Forty, fifty, or sixty years old is not too old to learn something new. It may prove difficult, but that's a feature, not a bug. You'll learn more about your limits, your misconceptions, and your strengths, and through the process of personal development, you'll become better in immeasurable ways. Who doesn't want that?