As an over 40 person who once coded full-time I can provide some insight. I had heard while in my thirties from older managers that there was a point when they no longer wanted to write code. After reaching this point myself, it appears somewhere around 40 (41 for me) the desire to write code goes away and is replaced with a desire to architect and/or manage.

However, there are exceptions. These are mostly lower level developers with years of experience in technologies that are really close to the metal. In fact, I just hired one.

Another aspect to consider is how the act of coding has changed. 20 years ago, it was about making cool things from scratch. Today, it is more about following a set of directions mandated by a framework or design pattern.