I think we could safety say that the first studies started a generation or two ago; things like this normally take a long time to end with living subjects. And each batch is taken as young teenagers with reaching puberty so that the change isn't as drastic. So Anastasia could easily be a hundred years old if we went for that and was the first that Zero came in contact with after being kidnapped. He wouldn't have seen her often before learning to control his powers, though as he proved an asset the two could have trained together. 

Would it be weird to him that she never ages?