One thing I did a lot of during my comparatively fallow, unproductive period was to memorize a lot of poetry. I picked up my copy of Robert Frost's poems from my office today to bring home, and found poems that I didn't even know I knew. I could look at them again and almost instantly re-memorize them. And I don't even like Robert Frost!
Memorizing that much changed my whole relation to my subject matter. When I say that I know a poem I mean that I've memorized it at one time, not just that I know of its existence. Insight have come from this level of knowledge, though I didn't need to memorize anything. I'm sure many scholars of poetry don't bother.
One thing memorization increases (possibly) is working memory, "the ability to actively hold information in the mind needed to do complex tasks such as reasoning, comprehension and learning" according to this wikipedia article.. I don't have any proof of that, but that's my own experience. For example, I can read an article once, then the next day look at it and write a reader's report. I remember everything that is wrong and right with it and don't even need to look back at it once I get the quotes I need for the report. I'm pretty sure my working memory was not this good even when I was younger, in graduate school.
The mind does decline in pure speed with age, but certain tasks can actually become easier.