I'm forty-two and an independent developer and my rate has gone up every year. If, as the article suggests, you allow your skills to go stale, then yes you will find it hard to get work. Duh.
If you keep yourself up to date and manage your career like any other field, you'll do fine. This doesn't mean you have to spend all of your free time training yourself on the next new thing, but you find work that involves newer technology and you learn how it works on the job. If you have a reasonable amount of curiosity you'll do this anyway.
It's worthwhile to talk to recruiters now and then to learn what skills are pulling in the top rates in the market. You may not want to pick them up (no matter how valuable, I simply won't do SharePoint), but you can find out where the market is heading. Networking with recruiters and colleagues is priceless, and it doesn't take much more than an extensive LinkedIn profile.
Personally I find I'm most effective when I switch back and forth between architecture (which emphasizes soft skills and leadership) and hands-on development (to keep my technical skills sharp). It's fun, challenging, and based on my experiences in the market, highly valuable. I try to cover as much ground as possible so that I'm as marketable as I care to be. Also, I don't commit to a particular technology/process/tool as if it's the "holy grail" of development. These things are like fashion and you need to roll on to the next new thing as it comes, even though it may be worse than the technology that it replaces.
Stay humble, stay curious.