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 googled it too, and got hundreds of references for the exact quote. I removed the quotation marks, and got 1,200,000 results.