CTListener said:
I kept a diary for Arbitron (pre-PPM) a couple of years ago, when I was 54. I told them how old I was, I mentioned that I listened a lot to XM, out-of-market stations via the internet, even shortwave -- they still mailed me the diary, but said I could leave the shortwave stations out. ;D I was just re-contacted by them last month and took that preliminary interview again but never got a meter.

Maybe I'm too old at 56.
Ratings measure everyone age six and above. PPM is more household based, and unless all members of a household or dwelling unit participate, they are not accepted.
Arbitron does not care what you listen to or even whether you listen at all. Their job is to measure a representative cross sample of the population, which will include heavy user and non users and everything in between.
On the other hand, a radio station will have a very specific research target as to age, amount of listening, gender, ethnicity, etc. Anyone outside this target will be rejected.