A puzzling fact about the Mars meteorites is that they almost all aside from ALH84001 have young geological ages:

New Clues About Martian Meteorites Baffle Scientists
"CHICAGO -- Analysis of new Martian meteorites is confounding planetary scientists with clues that simply don't add up.
"Rather than clearing up existing questions about the Red Planet, results from the new meteorites seem to be opening up a Pandora's box of questions about Mars. Evidence from the rocks doesn't seem compatible with one of the most trusted scientific conclusions about the planet: that the vast majority of the Martian surface is billions of years old."
http://www.space.com/scienceastronomy/solarsystem/martian_meteorite_000830.html

William Hartmann has proposed a possible explanation at the 32nd LPSC:

Watery Tales From Texas
"Hartmann also proposed an explanation for the puzzling fact that, so far, all the Martian meteorites recognized on Earth are made of lava only a few hundred million years old -- except for the famous "ALH84001" supposedly carrying evidence of Martian fossils, which is fully 4.5 billion years old.
"Where are the large number meteorites that should have come from the intermediate-age materials that cover most of Mars' surface? His theory is that Mars' ancient surface, in most areas, was pulverized long ago by the rain of small meteors onto it, and that this pulverized surface served as a perfect medium to contain Mars' near-surface ground ice, so that it consists of "ice-rich soils, loosely conglomerated sand and gravels, sedimentary layers, conglomerates, and sandstones" -- all of which are either crumbly enough that they cannot survive being blasted into space by the giant impacts that launch Mars meteorites to Earth, or else are "sedimentary rocks that would not be recognized by meteorite collecting parties".
http://www.spacedaily.com/news/lunarplanet-2001-01a3.html

However, the deep sediments on Mars have been proposed to cover the younger northern hemisphere. The older highly cratered southern hemisphere you would think would generate older meteorites.

I was therefore very interested to read this post on the bbs by Robert:

These ARE the weakest links
"One factor in the equation that I haven't seen mentioned is that panspermia in a system of planets aligned roughly to the ecliptic would depend on near-equatorial impacts. Higher-latitude impacts, I'd think, would impart an out-of-ecliptic vector. The resulting inclined orbits might make the ejecta inaccessible, or at least prolong the time before capture (playing against "6. Microorganisms can survive the ionizing radiation of space." and calculations of survival versus exposure time). If the orbital calculations don't already include this, the odds go down again."
http://www.habitablezone.com/space/messages/12915.html

Then the reason why so many found Mars meteorites are young is that they stem from equatorial regions on Mars. It is known that the equatorial Tharsis volcanoes have experienced geologically recent eruptions. Perhaps most of the Mars meteorites stem from these lava flows.

Bob Clark