When adding a quota question and you have an existing quota question selected, these quotas will be interlocked. For example, if you have an age question selected when you add a gender question, then male and female will be added for each age category (so you will be able to specify quota targets for each gender within the age categories).
In addition, you can drag and drop quota questions to produce the same result: dropping a gender question onto an age question would result in the two gender categories being added for each age category.