I'm planning to buy a used car. Assuming both cars have had no major
body work and have been maintained on schedule, is it better to buy an
older car with fewer miles on it (e.g. a 2000 or 2001 with 15,000
miles), or a newer car with more miles (a 2004 with 35,000 or 45,000
miles)? My tendency is to go for fewer miles since that's less wear on
the car, but I don't know how to factor the age of the car into the
bigger picture.

I don't think there's any good formula for calculating age wear & tear
vs milage wear & tear.  The best advice I can give is to know the blue
book value of each car for a good comparison (the blue book value
takes mileage and age into consideration).

Here's the Kelly Blue Book site:
http://www.kbb.com/

Hi christel,

It all depends on how you will be using it. If you will be putting
less than the average number of miles on it per year (say about 10,000
or less), then you can go with the newer car with higher mileage. If,
however, you'll be racking up the miles, then go with the older car
with the least mileage. In either case, in a few years, the
mileage/age will reach the "average".

Regards,
hummer

A car can only travel a certain number of miles before it is done. 
Call this amount of miles "X".  When that car has already traveled
30,000 miles, it now has a useful life remaining of X-30,000, which
can be significant.  If the car has been maintained properly, I would
go with the older car with less miles.