Here is an interesting idea: Our wrinkly green Muppet friend is not an alien as he appears to be. Instead, he is fully human. His wrinkled green appearance is a side effect of one of a Jedi's powers: lifespan extension.

Yoda started out 100% normal human Jedi, but The Force takes a price for extended life. He became small, wrinkly, and green. Perhaps his strange grammar is also a side effect, or maybe it is just an eccentricity that developed after 900 years.

Probably, not many Jedi chose this path. First off there is the obvious problem of becoming a Muppet. Secondly, it is probably frowned upon by the Jedi Council - a Jedi is supposed to accept death so that he can become a specter/disembodied voice. Yoda, however has a good excuse. He needed to stick around to train Luke Skywalker. Notice how Yoda dies as soon as he finishes with Luke.

I do have three pieces of evidence to present for my argument:
  • Skip to the end of The Phantom Menace, to the celebration on Naboo (don't worry, you're not missing much.) There will be a shot of the Jedi delegation, including Yoda, and behind him, a guy who looks to be half-human half-Yoda. Perhaps he is only 500 years or so old.
  • Observe the disfiguration of the Emperor's skin. This could be the early stages of Jedi lifespan extension. You could make a claim that that is just the corruption of being a Dark Jedi, but Vader's skin, while pale and sickly from being in the suit all the time, is not disfigured in the same way.
  • "When 900 years you reach, look as good you will not!" - said to Luke. Is this implying that Luke could also live to be 900?