When CrowdTap members sign up, we capture a series of demographic data points that are then built into their user profiles. These built-in demographics allow you to target by any combination of these data points up front or use them to cut your data on the backend.
For the demographics that can change (like marital status, parental status, etc.), those are asked once per quarter, so that respondents' profiles will stay up-to-date, even as they have life changes.