And if you think that the answer for "your brain stops working" is to go into management, all I can say is that I'm glad you're not my manager...
In my case, I'm pretty sure that I wouldn't be better than the ones I've been under (maybe I would be better than one or two, but not the majority). I'm also more likely to get fired as a manager, due to a tendency to say what I think.
If you're a better manager than the managers you've been under, you may be of more value to your company as a manager (even if you hate it). I may be selling myself short, but my value as a manager could be less than zero...
Yes, when you die.
Children are much better at learning new spoken languages, but by your logic no adult should even bother trying to learn a new spoken language.
Wrong. I've been at it for 30 years. It's not all roses, but it's still pretty good. The key is to be doing something where 30 years of experience is worth more than 5 years of experience. I'm in embedded systems, and the experience matters there (to enough people, even if not to everyone). Web programming? I'm less convinced that it matters there.
> Move to a management position quickly...
It is my explicit career goal to never become a manager. I've seen what that looks like, and I don't want it. I might make more money if I did, but it's not worth it.