My opinion: it must be a social hobby. The solitary task of software development may be isolating. What retirees miss post-employment isn't the work. But the colleagues and "work family" they fostered over perhaps decades.
Creative Coding. Maker Scenes. Art Therapy. All proven beneficial. Can also segue into art collecting or angel investing or philanthropy as a second career.
This is an interesting space with lots of possibilities for solutions. Best of Luck ;)