As a Scientist (I study brain computer interfaces for prosthetic applications), I don't see the lead up time as a problem. The specialization and detailed study required for a person to make important advances takes up till about 30yr old. After that you have a few years to find a position, set up a lab, start doing research under your own supervision, find funding etc. before you can really start producing good stuff. 35 is bang on for when I'd expect to be hitting stride.
The problem I see is that productivity tapers off after 15 years. Do people burn out? Do their brains stop functioning the same way? are the 15yrs of success just riding on ideas they had in their 20s? Can we keep good scientists productive longer? should we be shuffling them to admin roles faster to let bright new minds take the helm? Do we need to encourage risk taking by more established researchers?
All of that aside, I think that part of this may be that people tend to keep working in the fields they made their early seminal work in. Frequently a seminal work will take several years to digest through the community before it's recognized, and then several more years before the field has adapted enough for the next big breakthrough. This means that someone bright enough to make a significant contribution early in their career, is less likely to make them later on.
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.