Incorrect, George Romney was still in Mexico even up to his teenage years. He and his family did not arrive (back) in the US until George was a teenager. And, do we have any evidence that George Romney was actually “naturalized”...since he was officially a Mexican citizen when he arrived in the US
As the son of American citizens born abroad, Geo Romney would have been naturalized at birth under the US Code. He definitely had no business running in 1968, but his son Mittens is indeed eligible.