In the comments on the quit putzing around post, someone asked during what years is your mind at its sharpest. To a first approximation, before you're 30. That's worth emphasizing, since I don't think it's well known, and learning of it certainly got me going much faster in my studies. *
For a brief overview, search Ian Deary's Looking Down on Human Intelligence at Amazon, with the search phrase "cognitive ageing" (yes, with an "e" in "ageing"). Then navigate to the following pages for some sobering graphs: 224, 225, 228, 231 (or read the whole chapter if you have library / university access). What you see are two separate patterns for crystallized vs. fluid intelligence: the former tends not to decline until very late in life, while the latter begins a steep decline starting around age 30.
Since crystallized intelligence doesn't diminish for most of adulthood, your ability to acquire and store facts is not a limiting factor on your ability to contribute original insights to whatever it is that you do (assuming we're talking about smart people already). But because fluid intelligence -- your ability to reason through unfamiliar problems -- declines pretty unforgivingly at 30, you must exploit as much of your 20s as you can to come up with your Big Contribution.
Technically, this is true more for the fine arts and sciences, which make large demands on your fluid cognitive abilities, and not so much for anything about which you'd say, "OK, this isn't exactly rocket science" or "Well, it's not like I'm composing a symphony here." Still, in your early 20s you don't know exactly what you'll end up doing in your 30s, so it's better to prepare while learning new things isn't so difficult. That's the basic insight and argument underlying the idea of a standard curriculum for young people.
For example, while you could get a PhD in science or engineering and then read a lot of history in your 40s or 50s, it wouldn't be possible to get a PhD in history and then at age 40 or 50 learn intro calculus, genetics, and so on. It wouldn't take just a few months for someone else to teach you -- it would take as long as it would for an undergrad to learn, but now even longer since your mind isn't as sharp as it was at 20-25. And obviously if you remain pretty clueless about the basics, you won't get anything above that, nor be in a good position to offer an original thought that incorporates math or science. That's missing a lot: the accumulated knowledge we have about how the world works. And as Steven Pinker shows in The Blank Slate, an ignorance of the sciences of human nature has been the bane of many an idea about how society ought to be organized. Just one example of why it pays to study this stuff early on.
True, you don't need abstract algebra to understand any of the results that Pinker discusses, but I didn't say you had to do the equivalent of what a math major at MIT would do. Even a familiarity with basic statistics will go a long way to understanding such results, as well as give you the right weapons for intellectual self-defense, for when you come up against a know-nothing know-it-all, unfortunately a common occurrence.
So, like physical attractiveness and athleticism, raw fluid intelligence peaks during your 20s. By 30, your brain figures that it's learned most of whatever difficult concepts it must learn, so that continued investment of resources in fluid smarts could be better spent elsewhere. And as I mentioned in the review of The 40 Year Old Virgin, research shows that your personality traits remain remarkably stable after 30. The major tumult of life is done by that point, and you should have found your place in the world by then, so that significant further change would be somewhat pointless.
* In some cases, you could be a late-bloomer, in that you might have no trouble with the GRE, Miller Analogies Test, or other cognitive abilities test you take for graduate-level schooling, whereas you might have done well but not great on the SAT at age 18. If so, then you can probably assume the deterioration process won't begin until you enter your mid-30s. I think this is more likely if you are part or fully East Asian -- they tend to mature more slowly and live longer.