".. However, StackOverflow’s survey on age shows that there are just about 13% of developers after 40. Where are the others?" Obviously not on StackOverflow. Perhaps they have enough experience and knowledge to not need the site because they either know how to read tech books and sites to get the info they need when they have a problem using pre-existing docs, or conversely, are fully able to debug issues themselves.…
I don't understand how someone can "have enough experience and knowledge to not need the site" . I use StackOverflow practically all the time. I always think to myself, "yep, this (Wikipedia, YouTube and... yah HN) is what the internet is for" . But maybe I'm just too young. (I'm 47 ;) )
I'm not saying that either of these is correct, but they could be plausible explanations.