Free, white and twenty-one

Ok ya'll. I googled this and find no reference. Does anyone know the origin of the phrase "free, white and twenty-one?" I think it's Southern US derivation.

perhaps it is the opposite of "young gifted and black"?

L

I am not 100% on this but I think it is from a Hollywood film, either "I Am A Fugitive From A Chain Gang" or "Gone With The Wind".