How can I target a specific sample population such as only females or people between 30 and 40 years of age?

There are three options: 1) You can state the participation requirement in the project description and ask these questions at the beginning of a project for confirmation. You can then filter people who did not satisfy your requirements; 2) You can filter registered users who were willing to share their demographics; or 3) You can create an unlisted project and invite a preselected group of people to play.