I'm 57 and I "Can't Stop Coding" as I'm still having too much fun. Whenever I stop having fun... I change programming jobs. I wrote about this in a blog post (http://corgibytes.com/blog/2016/12/06/getting-old-er-in-tech...) that was widely read. I will say, however, that I have seen a lot of developers become obsolete mostly because they stayed with the same company (and programming language) for too long or ignored opportunities to stay abreast of technology.