I think, keep in mind how the makeup would be used. If they are pageant girls, or if they are in theatre, then of course they will get well-acquainted with makeup, perhaps at a younger age.
If we are talking about makeup to go out, I don't think it's necessary for a pre-pubescent girl to be donning such things for public wear. It's fine to play pretend or dress up while mommy is getting ready, but that's as far as it should go.
When they hit middle school/junior high, then maybe it would be appropriate to get them started on the basics: foundation, a lipgloss, nail polish... nothing too distracting... they are still young girls at that point. You don't want anyone mistaking them for high schoolers and older.
I guess by the time they hit high school, if they are interested in makeup at all, they will probably already be wearing a full face all the time (but of course, it is up to you to set the ground rules about how much is appropriate to go to school.)
I don't have my own daughters yet, but I would make the inference that it depends on your own value system, and what you think is age appropriate. It doesn't really matter what anybody else says, you have to lay down rules that work for your household.