How early can they do it?

While delving into Geogebra I feel somewhat obligated to give Desmos its fair chance. I decided today to just plot a triangle with movable vertices.

I did achieve that goal, although I doubt I could get a 3rd grader to follow along, I suspect the third grader could do it in GeoGebra.

I am looking for advice as to a user-friendly means of labeling the vertices. Such labeling occurs automatically in GeoGebra.