A fast food feminist can come from any walk of life. It is an inclusive category of person - you can be one at any age, you can be any sex, you can like any sex or even not like any sex at all if that is how you like to be.
You must be engaged in three things in order to be a fast food feminist. You must be engaged in fastness, you must be engaged with the food you eat, and you must be some sort of feminist.
In order to be categorized as some sort of feminist you must be interested in the welfare of those who are feminine. In this case when one is speaking about someone who is feminine one is speaking about women grouped by gender or by sex who live in the real world - one is usually not speaking about a concept or an idea or an ideal. Some sort of knowledge is presumed to be held as a feminist about the history of women as a group - from times long ago up to the closer-to-present time, when now in some places women are allowed to legally be considered people and not chattel or possessions.
In order to be categorized as being engaged with the food you eat you must be willing to look beyond simple teeth-gnashing hunger into the more expansive categories that engaging with food involves. Memory, culture, social mores and psychology are just the tip of the iceberg (those things that exist in the North Pole, not the lettuce though certainly we can discuss lettuce later!) along with an interested and discriminatory palate that does not consider a plate of food a trough to be analyzed mostly for size, quantity, and price.
In order to be categorized as being engaged in fastness the thought of how to best spend the limited hours of life must consume at least part of the thinking process each day, with the pros and cons of "faster is better" or "slower is more fulfilling" or "finish what you started" or "first things first" or "how can I talk on a cellphone, send IM's, watch the news on TV, keep up with my work, and keep my hair perfect at the same time" sometimes entering into the thought process to be sifted, sorted, and ultimately decided or not.
That just about describes what a fast food feminist is.
It's not only about "fast food".