The surveys look for specific targets in terms of age, gender and so on. If your answers reveal you do not match that target, the survey will end.
This does not mean you have to lie when completing a survey.
Surveys are made in such a way to easily spot fake answers and fake data.
On the contrary, one of the ways to have more surveys "fitting" your profile (and therefore complete more surveys) is to answer as honestly as possible.
We know it's frustrating to spend some time on a survey and then get suddenly disqualified, but do not worry: different surveys are looking for different people and demographics, and our team is working the whole time to increase our survey inventory to offer you more surveys to complete.