QUESTION:
What's an estimate of the number of craters on Mars? Are they older than the
ones on Earth?

ANSWER from Jeff Plescia on November 18, 1997:
There are literally thousands of craters on Mars and they range in size from
small features a few meters across to large basins such as Hellas Basin
which are between 1300 and 4200 kilometers across.

The Earth was cratered just as much as Mars was.  The difference is that
almost all of the craters of the Earth have been eroded away or destryoed by
sea floor spreading.  So, the oldest crater observed on the Earth is younger
than the oldest craters on Mars.