How do you know what "age appropriate" is?

How do you know when you should tell a child something and when it’s too soon? How do you decide? Is it based on any research or is it just by intuition and what you feel like (or don’t feel like) talking about?

It is my sense that “age appropriate” is a cultural thing, not any kind of scientific thing. In different cultures, I’ll bet there are different notions of what is age appropriate. It just depends on the pathology of the culture.

If it is cultural, and isn’t related to repeatable studies, then how do you feel about it? Is making choices based on cultural norms appropriate? Or do we need to study things and make our choices based on what actually works?