Hey there
I dont think it's impossible for a 6 year old to have a full, but belive me he/she will lose it and have to get it back. If you are a gymnast you know what I mean

It takes years to master proper technique in gymnastics. Its not like you just walk into a gym and start throwing everything.
Oh I even had an example, when I was training still in gymnastics there was this 5 or 6 year old boy. He was the cutest thing ever. He could throw a full. Needless to say when the boys coach started on(he was an Olympian from Poland) would not let him do fulls anymore. He brought him back to the basics.
You see in order to let us say really master a double full, its not as smiple as being able to go from a bacjtuck one day to throwing a double full(beleive me I have seen many of my cheerleaders try) to have the proper orientation and awarenesss that one needs, once you have that you can easliy throw that skill on floor, off beam, and off bars. Its really mastery of skill. MOST 6 year olds I have seen that can throw certain skills seem to lose most of those skills after a few years. That is why certain basics need mastery that cannot be fullfilled at age 2.