EDIT: To be more precise, I cannot image that I will write a program after my retirement. To me, writing ephemeral programs is the opposite of the artful activities described in the article.
What better time to start your own software business than when you've retired?
The beauty and curse of working with bits is that they're as flexible as you want them to be.
Programming satisfies part of my desire for artistic expression. I don't plan on quitting until I'm dead - I'm more passionate about computing, from high level abstractions and algorithms down to individual lines of code, at 39 than I was at 19 or 29. And I know a lot of people older than I who are way better than I am, and just as gung-ho as they were 20 or 30 years ago.
Why not?
[1] http://en.wikipedia.org/wiki/Age_Discrimination_in_Employmen...
[2] http://www.equalityhumanrights.com/private-and-public-sector...
It's not at all 'discrimination'. BTW, I'm not young any more. Programming just isn't worth it.