I find the study to be highly suspect. Looking at Stack Overflow means that you're only looking at people who are looking to learn & grow. It could be perfectly possible that programmers "weed out" of career advancement as they age and stay stuck with the same skills but those same programmers never think to create a SO profile.
I have reservations about the study, but for different reasons. I suspect there is a correlation between age and the perceived value of participating in online discussions. I also suspect that there is a correlation between age and the perceived value of that person's contributions to an online discussion.
I suspect that in Stack Overflow discussions which discourage lols and memes, older programmers are more likely to find an intrinsic value from participation and are likely to be better able to communicate their responses clearly (just by having more experience writing).
Joel Spolsky created a site he (as an over forty programmer) deemed constructive. Those sharing his general experience are more likely to share his views about what is constructive.
The same might be approximately true for HN (PG is about a year older than Joel Spolsky). But the less ruthless editing of HN may play a role.
