Question about ratings - How do they figure out how old the viewers are?

[Samurai]

Everytime I see ratings figures I always see age demographs, like 6-11, etc. But how do they figure out the age of the viewers watching? For instance, I read Toonami had some troubles in the past with some of it's shows attracting an older audience and not enough younger viewers. But how do they know that? Or are they simply just guessing? I'm curious...

[lostrune]

Everytime I see ratings figures I always see age demographs, like 6-11, etc. But how do they figure out the age of the viewers watching? For instance, I read Toonami had some troubles in the past with some of it's shows attracting an older audience and not enough younger viewers. But how do they know that? Or are they simply just guessing? I'm curious...
The Neilsens survey asks the participating viewers. Seriously.
Whether it's a manual "logbook" or an electronic box, the participants are asked to fill in some info, including ages. That's it. Just like any other survey. The surveyor asks the surveyee.