This all is true, but it's actually a lot worse for non-developers. Sales, most middle management jobs, generally MBA's are utterly worthless after 50 and no-one would hire them even if they have skills (they apparently didn't prove them otherwise they would be big successes before). And that's only talking about people who work with their brain; working with your hands is worse than that even.

I think the most important thing to do; if you have a job make sure you keep it after 50; don't toy around or give anyone reasons to drop you. And make sure you have a huge network; make sure different people ask you for advise or ask you to work for them at least a few times per year.

Besides that there is not much you can do if you are not outwardly hugely successful at what you do; if you are a dev, that means specialize, write books and get them published about what you specialized in, create workshops and seminars, even next to your job but especially at your job.

I must say that it became slightly easier though for people who are willing to reinvent themselves a bit; besides making sure that you have medical private insurance just always (and yes, you can afford that when you are young; make sure you keep it going even if your job provides insurance as well; and when you are young you feel invincible until you get something which could've disabled/kill you; after that, if you didn't have insurance before it, no-one will ever take you so it's important not to think you are invincible ;), you can work remotely and then age doesn't really count at all. I have many 'older' friends who work online as coder for people they have never seen or met and they are doing fine. No they won't get the insane amounts they were once used to, but maybe you shouldn't get used to that wastefulness anyway; they can do what they like (which happens to be coding) until they die basically and make enough money to pay for health insurance etc. Most of them are 60-somethings who make enough that way to lead a really good life.