[butchdave]

Since piston rings are exandable, they do compensate for the wear both in themselves and the cylinder walls, so its only as they get older that compression is lost. Compression can also be lost through wear in the valve seats and valves.

One thing that hasn't been touched on but relevant to the question since most people here are petrol heads is that the fuel will also make a difference. Most diesel engines wear more slowly than gas, reaching higher mileages before suffering compression loss. Though I have to say that as I sit here and write that, any explanation for that completely escapes me!