Psychology

Identifying Participants Variables, you can choose age, occupation, gender, ethnicity, socioeconomic status, literacy status, etc. as your variables.

Example - Research Question: How does the age of a person affect the way they cope with stress (or what stress coping mechanisms they use)?  You may pick age, occupation and gender as the variables.

If it is an experiment on the effects of sleep deprivation on test performance;  factor such as age, gender, and academic background may have an impact on the results.