It is an approximation of an age group which holds features that you find most attractive. It isn't really anything solid, and probably changes over time or on a case by case basis.

For the record, my primary AoA is somewhere around 4-12. However, it occasionally goes lower (about age 2 the lowest, extremely rare though), and in reality has no upper limit. But I have been most attracted to girls who tend to be in that age range. It has nothing to do with "Oh, she's six years old, she's really cute."

I don't really see it as ageist, nor do I see any real reason to stop using them. I also don't see it as stereotyping.

e.g. "Most girls I have been most attracted to have been within the ages 4-12."

Is this stereotyping? I personally don't think it is. I see it as an observation of personal experience.

Compare to: "Most black women I have met were loud."

Did I just stereotype black women? I don't think I did. I merely made an observation of my personal experience with them.

As opposed to: "Most black women are loud."

That would be stereotyping.