QUESTION:
How old is Mars? 

ANSWER from Ted Roush on February 20, 1997:
One means of estimating the age on Mars is to count the different size
of craters seen there.  Based upon experience with the lunar surface and
the ages determined for the rocks returned from the moon, the greater
the number of large craters, the older the surface.  This technique has 
been used to estimate that some of the surfaces on Mars are quite old.

Another method to estimate the age of Mars is to determine the ages of 
the meteorites believed to have once been rocks on Mars.  This 
technique has been used to determine ages of about 4.6 billion years for
the oldest materials.  Thus the oldest rocks on Mars appear to be about
the same age as the 4.5 billion year old materials found on the Earth.