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It’s not all MIT degrees, exactly, but it is most. Most degrees require 180 units beyond the GIRs, which is what you get if you take all 17 GIRs at MIT and take four classes per semester. There are some majors which require more than 180 units outside the GIRs, though (as Piper is pointing out above).</p>
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Generally speaking, you can take any graduate course whenever you feel you’re ready. There was a CC poster a few years ago who took all grad classes his first semester, and I don’t think his department ever had a problem with it. Some classes will require you to get the permission of the instructor to register, but in general you’re welcome to take any course you want (other than limited-enrollment situations like some lab classes, seminars, etc.). </p>
<p>Of course, a large number of classes at MIT are joint grad/undergrad classes – it’s the same class (same professor, same room, same time), but undergrads register under one number and grad students register under another.</p>