One factor that I see contributing, besides "burning out" or "brains not working," is that older scientists are increasingly shunted into administrative / advisory roles. The scientists I know in the "over 50" category tend to devote more time to to things like being a department chair, or flying to meetings of national science advisory boards. I wouldn't say this is necessarily a bad thing; as a younger researcher, I love the fact that someone else is dealing with the boring political/administrative aspects of research, so I can put in time in the lab doing "creative" research. Older scientists haven't necessarily stopped contributing; instead, they're "behind the scenes" enabling the next generation of younger colleagues to be so productive. Right now, I wouldn't want to end up like that (a "senior" scientist more focused on high-level administration than the hands-on details of research), but perhaps my opinions will change in a few decades --- and I can certainly see a lot of contribution to the overall scientific endeavor by older scientists that wouldn't be counted by a simplistic "groundbreaking research papers" metric.