There is no such magic age recognized in the law. The best interest of the child is the standard.
The law regarding a child's 'right'* to choose is a matter for each State and jurisdiction. The judge in most States, not the child, makes the decision based on the best interest of the child. Although not a standard by any means, many States have begun to give 'consideration' to a child's declaration of custodial preference when the child reaches the age of twelve or thirteen, sometimes fourteen. There are even cases when children of age 9 are allowed to testify.
The judge is normally given almost unlimited latitude in whether or not she or he listens to a child and how much weight to give to the child's wishes. In short, there is no specific "age" but the younger the child the less likely for a judge to give the stated preference much weight.
Good luck to you.
Matters of custody and visitation are determined by analyzing the "best interest of the child" factors at Va. Code 20-124.3. Factor 8 says, "The reasonable preference of the child, if the court deems the child to be of reasonable intelligence, understanding, age and experience to express such a preference." So, there is not required age for a child to express his or her preference. It is a matter of how much weight to give the preference. Also, there are 9 other factors.
However, the bigger challenge will be for you as grandparents to overcome any objection from the parents. Va. Code 20-124.2(B) says in part, "The court shall give due regard to the primacy of the parent-child relationship but may upon a showing by clear and convincing evidence that the best interest of the child would be served thereby award custody or visitation to any other person with a legitimate interest."
The court typically requires proof by a preponderance of the evidence. As a third party, you would have the higher, clear and convincing, burden of proof.