A very recent Martian crater. Click for a larger version (~100k). Credit: NASA/JPL/University of Arizona

Pretty new anyways.  The crater is pretty small being only 18 feet (5.5 meters) across. This little crater was formed between January 2006 and May 2008.

The crater was found because this area has been imaged earlier with the Themis instrument and no spot was present.  Then more recently the CTX camera was being used to  survey the Martian surface, the CTX is used because it has lower resolution and therefore more area can be covered to look for changes.  When the same area was imaged and the photos compared a dark spot was noted.

Since the CTX instrument could only detect the spot but not resolve it because the crater is about the size of a single pixel, the HiRise instrument was used to take a hi resolution image of the spot.  The spot of course turned out to be the crater and the dark area around it.

Click the image for more of the area around the crater.  A larger version of the image can be found at the HiRise site.