The post below reminded me of something else I wanted to mention: I am 32 years old, and find that it is noticeably harder for me to grok new models and concepts than it was, say, five years ago. That is to say, it takes more time and more mental effort for me to understand things that do not directly relate to things I already know about. I worked at my current job for a month or so before I actually understood the underlying philosophy to what I was doing--I actually blogged about the meeting I was sitting in when it finally hit me what we were working on.
This is a well known phenomenon, I guess--it is generally understood, in academia for instance, that new schools of thought and modes of study come in with the new faculty and the old ones don't leave until the faculty who study and/or championed them retire. This is so well understood in mathematics that the Field's Medal, which is the Nobel Prize of the discipline, cannot be won by anybody over 35--it's part of the rules. But it's only lately become clear to me that it's not just that people get set in their ways, or like to stick with modes of thinking in which they're familiar, but that the brain is physiologically becoming fixed. The neural connections have already formed, and there just aren't that many more left to fuse.
Well, I'm depressed. I'll never win that Field's Medal now.