Well, the survey companies have to make money, too. They get paid by advertisers to find out particular pieces of information about particular demographics. So fairness is not the point, fitting the demographic is.
But Playata is likely not interested in telling the survey companies what kinds of surveys to run, and the survey companies are just taking their orders from the advertisers. It's not actually age discrimination, it's just marketing research.
What I object to is incompetence or fraud. If a survey maker is too stupid to get their qualifying questions out of the way quickly, they are wasting my time. If a survey maker intentionally asks for information, and then always finds a way to disqualify paying for my time, they are being fraudulent.
Perhaps others have different experiences. As I said, I got nothing out of surveys, so I stopped doing them.